Piet Mondrian, Composition II in Red, Blue, and Yellow, 1930 CONSCIOUS UNITY FROM THE TOP DOWN: A BRENTANIAN APPROACH Anna Giustina Forthcoming in The Monist 100 (2017) The question of the unity of consciousness is often treated as the question of how different conscious experiences are related to each other in order to be unified. Many contemporary views on the unity of consciousness are based on this bottom-up approach. In this paper I explore an alternative, top-down approach, according to which (to a first approximation) a subject undergoes one single conscious experience at a time. From this perspective, the problem of unity of consciousness becomes rather the problem of how we can distinguish a multiplicity of goings-on within our conscious experience at any time, given that it is unique. I will present three possible top-down approaches to unity of consciousness, which I call Priority unity monism, Existence unity monism, and Brentanian unity monism. Priority monism and Existence monism are defined in analogy with the homonymous metaphysical theories of object constitution. Brentanian monism retraces Franz Brentano's view on unity of consciousness, and is defined by appeal to some of his mereological ideas. I will argue that the latter is the best top-down approach to unity of consciousness. 1. INTRODUCTION It is ten to noon and I am sitting in a park, leafing through a catalogue of Mondrian's paintings. While I look at the photograph of Composition with Red, Blue and Yellow, I peripherally see the text describing the painting, my hands holding the book, and, beyond the book, some people and trees, and a fountain in the background. At the same time, I hear the fountain's water pouring down, girls chatting on my left, leaves rustling and children laughing, and other background noises. I have my legs crossed, and I am aware of that. Further, I am eating nut chocolate, tasting its flavour and feeling the little nut pieces in my mouth, and smelling the pleasant odour of the roses behind me. Unfortunately, I have a bad headache, so I am thinking that I should take some aspirin. All those conscious states occur simultaneously at ten to noon, and they belong to the same subject-me. They occur together, and I experience them as occurring together. 2 The conscious states I have at ten to noon also seem to be tightly related to each other. The picture of Composition appears to me as a unified image. Moreover, it appears to me as being part of the book I hold in my hands. It also appears as standing in a specific spatial relation with the text which describes it (say, on its left). The book itself appears to me as a unified thing, and as bearing spatial relations with me and the other objects in my visual field-for example, it is closer to me than the trees and the fountain in the background. The objects around me are unitarily perceived by me through different sense modalities-the fountain, for instance, is both heard and seen. My thought about aspirin is causally related to my feeling pain in my head. And so on. Not only all the conscious states I have at ten to noon appear to me as occurring together. They also appear as being intimately connected in such a way to form a unified state of consciousness. At any one time we simultaneously have a number of conscious states. It seems intuitively true that such conscious states are somehow unified. But what makes them unified? In virtue of what are all the conscious states one has at any given time unified? This is the problem of the unity of consciousness. Before attempting any solution to the problem, it is worth drawing some distinctions, and to clarify what exactly the phenomenon at issue is. First, what we are concerned with is phenomenal unity, namely the unity of one's phenomenally conscious states. This must be distinguished from other kinds of unity, such as access unity (the unity of one's access-conscious states), introspective unity (the unity of one's introspected conscious states), and cognitive unity (the integration and coordinated use of one's mental faculties, aimed at task accomplishment, problem solving, and interaction with the environment).1 It must also be distinguished from behavioural unity (the capacity to coordinate one's body parts in order to produce more or less complex actions), and neurophysiological unity (the unity of a set of conscious states which are correlated to or brought about by the same area of the brain).2 Second, unity of consciousness is unity of all the phenomenally conscious states of a subject, rather than a subset of them. Thus, it must be distinguished from both objectual and spatial unity, which are only forms of partial unity. Objectual unity is the kind of unity which binds together, say, your perceiving redness and your perceiving roundness, thereby enabling you to perceive a red ball as a unified object. It also enables you to perceive one and the same object through different sense modalities. Objectual unity is only partial unity because, usually, not all the conscious states one has at a time are directed to one and the same object. Spatial unity is the unity of conscious states which represent objects as belonging to the same space. It is merely partial because not all the conscious states one has at a time represent objects as being located in space, or as bearing spatial relations with other objects. My thought that I should take some aspirin, for instance, does not represent anything as being located in space.3 Third, a distinction must be made between synchronic and diachronic unity. Synchronic unity of consciousness is the unity of the conscious states one has at a certain time t. Diachronic unity of consciousness is the unity of one's conscious states over time. What I will focus on here is 1 A mental state is access-conscious if, in virtue of having it, its content is available to its subject for inferential reasoning, verbal report, and behaviour control. The distinction between phenomenal and access consciousness has been drawn by Ned Block (1995). 2 For a taxonomy of kinds of unity see Tim Bayne and David Chalmers (2003: 24-27), Michael Tye (2003: 11-15), and Barry Dainton (2003: 211-15). Bayne and Chalmers discuss quite extensively the difference and the relation between phenomenal and access unity in (2003: 27-34). 3 Another counterexample is, say, my consciously having a mental image of a kangaroo-for those who do not believe in cognitive phenomenology. 3 synchronic unity of consciousness. I will leave aside the question of how several conscious states are diachronically unified in one single stream of consciousness. Those who offer accounts of the unity of consciousness typically agree on the following weak unity thesis: WUT: All the phenomenally conscious states of a subject at a time are unified. Some of them also endorse the strong unity thesis: SUT: Necessarily, all the phenomenally conscious states of a subject at a time are unified.4 The unity thesis is widely accepted, for two prima facie reasons.5 First, it strikes many people as intuitively true. Second, it seems that cases of one's consciousness not being unified at a time are hard, if not impossible, to conceive.6 Perhaps a decisive argument for SUT has not been provided yet. However, much important work in that direction has recently been done (especially by Bayne 2010, who has defended SUT from a number of empirical objections).7 Providing such a decisive argument is not the purpose of this paper. In what follows I will assume that SUT is true. The purpose of this paper, instead, is to explore a new account of that in virtue of which SUT holds. Thus, I will focus on the following question: Q: What are the basic personal-level facts in virtue of which all the phenomenally conscious states of a subject at a time unified? Farid Masrour (2014: 325) rightly points out that this question, which he calls the 'Grounding Question', should be distinguished from the 'Structural Question': what are the necessary and sufficient personal-level conditions for unity of consciousness? The two questions are related, but their answers might not coincide. Consider the following example, offered by Masrour (2014: 4 This thesis is defended by Bayne and Chalmers (2003), and by Bayne (2010). 5 Although there are exceptions. Susan Hurley (1998) discusses the conceivability of partial unity of consciousness. Christopher Hill (2014) argues that, possibly, not all the conscious states of a subject at a time are unified. Some philosophers even maintain that consciousness is often disunified-see e.g. Dennett (1992), O'Brien and Opie (1998), and Prinz (2013). 6 What would our conscious experience be like were it not unified? It would be scattered in countless disintegrated mental states bearing no relation to each other. No experience of space would be possible, nor perception of objects. It would be impossible to see and hear, or smell and desire the same thing. We could not compare things, nor form judgements on the basis of perception. Moreover, in what kind of pieces would our experience be disintegrated? Perhaps the visual field would crumble into an unstructured scatter of colours. Dainton (2003: 209-10) seeks to envisage what one's conscious experience would be like were it not unified: "Suppose it were be possible to eradicate all trace of phenomenal unity from one's consciousness at the press of a button. [...] What would it be like? What would be left? Do entities such as individual conscious thoughts, smells, or sounds possess parts which could become separated?" He suggests that we might be able to imagine what would happen to one's visual field: it would probably crumble into countless coloured points. However, as Dainton rightly points out, the outcome would be even worse: "If we remove all trace of phenomenal unity each momentary point-color would be experienced all by itself, in total isolation." Moreover, in what pieces would smell, thought and desire crumble? What disunified experience would be like is hard to conceive. It might not even be conscious experience at all. As Bayne and Chalmers (2003: 55) point out, "[m]uch of the reason for accepting the truth of the unity thesis comes from the fact that its denial seems to be inconceivable, and perhaps incoherent." 7 Alleged counterexamples to the unity thesis are pathological cases in which phenomenal unity seems to break down. The paradigmatic case (which gave rise to the contemporary debate on the unity of consciousness) is the split-brain syndrome (see Nagel 1971). One strategy against those empirical objections consists in showing that, in the proposed counterexamples, it is some other kind of unity (e.g. access unity) which breaks down, and that phenomenal unity is preserved. 4 footnote 7). One answer to the Structural Question is (P): All the conscious states of a subject at a time t are unified iff they are parts of one single overall experience E. Now, there are three possible answers to the Grounding Question which are compatible with (P), namely: (P1) The unity of all of one's conscious states at t grounds both the existence of E at t, and the fact that one's conscious states at t are parts of E; (P2) The existence of E at t and the fact that one's conscious states at t are parts of E ground the unity of all of one's conscious states at t; (P3) Both sides ground each other. The Grounding Question seems more fundamental than the Structural Question, since any answer to it will tell us whether unity is primitive or it is explained by some other personal-level facts. Answers to Q divide in two classes. On one side are what I call pluralistic views, on the other what I call monistic views. Pluralistic views maintain that unity of consciousness holds in virtue of a special relationship which binds together all the conscious experiences a subject has at any one time. They thus adopt a bottom-up approach, and assume that: (i) at any one time t, a subject has a number of conscious experiences; (ii) all the subject's conscious experiences at t are unified in virtue of a certain relationship R. From the bottom-up perspective, a theory of unity of consciousness should explain what is the relationship R that makes such conscious experiences unified. Barry Dainton's (2000) view of unity of consciousness is a clear example of a bottom-up approach. He holds that "[a] state of consciousness is fully phenomenally unified at a given time by virtue of the fact that its constituent experiences are all mutually co-conscious*." (Dainton 2003: 216).8 Co-consciousness is the phenomenal relationship of being-experienced-together, which all the conscious experiences of a subject at a given time bear to each other. Such a relationship is basic, that is, it does not depend on any other personal-level facts. Accordingly, the notion of coconsciousness is primitive: it cannot be explained in terms of anything else. Monism about the unity of consciousness (unity monism for short) is the view, roughly, that a subject's consciousness is unified at a time t in virtue of the subject's having only one conscious experience at t. Monistic theories thus adopt a top-down approach. They deny that a subject has many conscious experiences at a time. There is no special relationship in virtue of which unity holds. Unity is simply due to the fact that at any one time the subject has one single conscious experience. Franz Brentano was no doubt a monist about the unity of consciousness. In Psychology from an Empirical Standpoint he writes that one's simultaneous conscious states "belong to one real entity. And it is this common membership in one real thing which constitutes the unity about which we are speaking." (Brentano 1874/1973: 161, italics original). At a first approximation, his view was that at any one time all of one's conscious states are unified in virtue of their belonging one single state of consciousness. The bottom-up approach seems to be the most widespread among contemporary theorists of the unity of consciousness. Accordingly, the question of unity of consciousness is often formulated in terms of how several conscious experiences stitch together. In this paper I will dogmatically adopt a top-down approach, and explore monistic views about the unity of consciousness. The main motivation for focussing on monism is that it has been less explored in the literature than pluralism.9 Although I am not going to argue for this here, monism might have some advantages 8 The star is in the text and is intended to distinguish Dainton's notion of co-consciousness-which is basic and sui generis-from other forms of phenomenal unity relation. 9 To my knowledge, the only explicit contemporary proponent of monism about the unity of consciousness is Michael Tye (2003), who calls his theory the 'One Experience View'. 5 over pluralism. In any case, in order to be properly evaluated, monism needs to be examined more fully. An analysis of its different versions is needed, as well as an evaluation of their strengths and weaknesses and of the possible solutions to their problems. The aim of this paper is making some progress on this matter. I will explore some possible monistic views about the unity of consciousness, evaluate them, and argue that one of them seems to be more promising than the others. As noted, monism is poorly represented in the contemporary debate on the unity of consciousness. A standard taxonomy of its possible versions is thus unavailable. It sometimes occurs that debates in different areas of philosophy have similar structures. In some cases, one debate has reached a quite more advanced stage than the other; the logical space of the less advanced debate may not be as crisply defined as that of the advanced one. In such a case, it may be helpful to draw an analogy between the two debates, and see whether an analysis of one can shed light on the other. 2. MIND/MATTER: AN ANALOGY There is a lively debate in metaphysics, concerning the composition of concrete objects and the relation between whole concrete objects and their parts. Peter Van Inwagen (1987) has influentially distinguished two questions: the General Composition Question (GCQ) and the Special Composition Question (SCQ). The former asks what composition is, the latter under what conditions composition occurs. According to him, SCQ should be addressed first. One possible answer to this question has the form: a plurality of concrete objects compose a further object iff they bear the relationship R to each other.10 Van Inwagen's SCQ seems to be the analogous of Masrour's Structural Question about unity of consciousness. However, arguably, those interested in object composition should not only provide its necessary and sufficient conditions, but also that in virtue of which composition occurs. They thus need to face the Grounding Question: in virtue of what do a plurality of concrete objects compose a further object?11 One possible answer is: a plurality of concrete objects compose a further object in virtue of their bearing the relationship R to each other. Those who attempt this kind of answer adopt a pluralistic account of object composition. Pluralistic answers to SCQ are widely considered to face some difficulties, which I do not have the space to discuss here. I will just mention two of them-arguably the most serious. First, it is hard to find a relationship which provides composition with both necessary and sufficient conditions (Van Inwagen 1987). Second, pluralists' criteria for composition often turn out to be vague. They often fail to draw a neat demarcation between cases where composition occurs and cases where it does not (Van Cleve 2008). What is relevant for our purposes is that such difficulties drove some metaphysicians to look for an alternative account of object composition. 10 Limit-case answers to SCQ do not posit any special relation. Nihilists claim that there is no relation at all among object parts, since a set of objects never compose a further object. Universalists, by contrast, say that there is no special relation, since objects are always related in such a way to form a further object. But these extreme answers face some difficulties. Nihilism leads to the non-commonsensical conclusion that objects as we commonly perceive them, such as plants, animals and human beings, do not exist. Universalism ends up with a hardly acceptable claim as well, since it entails that the object composed by, say, my glasses, Notre Dame Cathedral and John is a full-fledged concrete object, on a par with a tree, a pen, or me. 11 And indeed a vast literature on grounding has been flourishing quite recently. See e.g. Fine (2001), Rosen (2010), and Schaffer (2009). 6 Accordingly, some monistic views have been recently formulated-or, better, recovered. At a first approximation, monism is the theory that there is only one basic concrete object. For a concrete object to be basic means that its existence depends on no other concrete object's existence-that is, no other concrete object is ontologically prior to it. There are two main kinds of monism in the current literature (Schaffer 2015): Existence monism and Priority monism. Existence monism is the view that there exists only one concrete object, namely, the whole universe.12 The universe is basic because there are no other concrete objects on which its existence might depend. The unique existing object does not have any proper part. What appears as a multiplicity of distinct concrete objects is in fact the result of the way in which the unique existent object is complexly structured. Priority monism is the view that there is one single basic concrete object, the cosmos, which does have parts.13 Its parts, however, are not themselves basic. Their existence depends on that of the whole cosmos. Recently developed monistic accounts of object composition have thus opened new promising routes to bypassing SCQ. Might things be similar for unity of consciousness? A first pass toward an answer may be presenting a range of alternative versions of unity monism. I will suggest that this can be partly done by drawing an analogy with ontological monism. Accordingly, two versions of unity monism can be identified. Priority unity monism is the view that one's conscious states are unified at t in virtue of their being (non-fundamental) parts of one and the same basic conscious experience. Existence unity monism is the view that one's consciousness is unified at t in virtue of one's having only one (partless) conscious experience at t. Sections 4 and 5 will be dedicated to clarifying both Priority and Existence unity monism, by appeal to the analogy with their ontological counterparts. I will evaluate them, and highlight their strengths and their weaknesses. In section 6 I will suggest that a third option is on the table. The latter, which I call Brentanian unity monism, is inspired by Brentano's view on the unity of consciousness and based on some of his mereological ideas. I will argue that Brentanian unity monism, while preserving their strengths, does not incur the difficulties which affect the other two. 3. THE DISTINGUISHABILITY PROBLEM As noted, unity monism is the view that one's consciousness is unified at t in virtue of one's having one single basic conscious experience at t. A straightforward prima facie objection comes to mind: how can monistic views account for the multiplicity of goings-on that characterise our conscious experience? At any one time our conscious experience is extremely complex. A great number of conscious mental states seem to occur at the same time. They appear unified, but still somehow different from one another. Recall the scenario I outlined at the beginning of section 1. My seeing Mondrian's Composition, my hearing the fountain, my tasting chocolate, and my thinking of aspirin are mental states that seem clearly distinct from one another. Indeed, although at ten to noon it is unified with the other conscious states, my seeing the photograph of Mondrian's painting might be the only mental state I recall of the overall experience after a few days. In order to recall it independently not only of my memory of the overall conscious experience at ten to noon, but also of the visual scene it belongs to, the photograph must already be experienced as distinct from the rest. In other words, I must be able to single it out at the very moment I am having the overall 12 Perhaps the only contemporary proponents of Existence monism are Terry Horgan and Matjaž Potrč (2000). 13 The main contemporary proponent of Priority monism is Schaffer (2010). 7 experience. This is what I call the distinguishability problem. How can I distinguish different conscious states at t-and thus be able to single out one particular conscious state-if I have only one experience at t? To be viable, any version of unity monism must have a solution to the distinguishability problem. 4. PRIORITY UNITY MONISM Priority monism has a straightforward solution to the distinguishability problem. Although a subject has only one single basic experience at t, that experience has proper (experiential) parts. Those parts, which coincide with the different conscious states the subject has at t, are not themselves basic. Still, they are real. Thus, one can single out different conscious states because they are distinct parts of one's overall unified experience at t. Priority unity monism is defined in analogy with ontological Priority monism. An explanation of the latter might be helpful for better understanding the former and its response to the distinguishability problem. Jonathan Schaffer (2010) is the main proponent of ontological Priority monism. According to him, there is just one basic concrete object, namely, the whole cosmos. The cosmos has proper parts, but the parts metaphysically depend on the whole. Within this framework, pluralism is not the thesis that there are many concrete objects (this is true for Priority monism too) but rather that there are many basic concrete objects.14 As Schaffer himself points out, his theory relies upon some assumptions (2010: 33-38). First, a threefold mereological assumption: (i) there is a cosmos, that is a maximal concrete object; (ii) the cosmos has proper parts, namely all the other existing concrete objects; (iii) the cosmos is not identical to any plurality of its proper parts.15 Second, a twofold metaphysical assumption: (iv) there are metaphysical priority relations among concrete objects, that is, the existence of some objects depends on the existence of others; (v) priority constitutes a well-founded partial ordering, that is, it is an irreflexive, asymmetric, transitive relation, and there is something (a foundation) on which everything else depends (the dependence chain is not endless). Hence, according to priority monism, the existence of all non-fundamental concrete objects depends on the existence of the unique basic concrete object-the cosmos.16 Schaffer defines priority as an irreflexive, asymmetric and transitive relation that brings about a hierarchical order among entities, such that more fundamental entities ground less fundamental ones. The latter derive from, depend on, and exist in virtue of the former. Since the dependence chain must come to an end, there must be an ultimate basic entity in which all other entities are grounded. Priority monism about unity of consciousness is defined in analogy to ontological priority monism. Accordingly, a subject has only one basic conscious experience at t. All the conscious 14 Schaffer (2015) stresses that the debate between priority monists and pluralists should be seen not as a debate over what exists, but rather as a debate over what is fundamental. 15 Composition must be distinguished from identity: "If the one literally is the many, then monism and pluralism would no longer be opposing views-indeed both 'sides' would turn out to be right." (Schaffer 2010: 35). 16 It follows that the properties of concrete objects depend on those of the whole cosmos (for example, my shirt being red depends on the cosmos being as it is). 8 states she has at t are proper parts of the experience that encompasses them, but they are not fundamental-they depend on the overall state.17 Its main thesis may be formulated thus: PUM: All the conscious states a subject S has at t are unified in virtue of S's having one and only one basic conscious experience at t. The different conscious states S has at t are non-basic experiential parts of the overall experience.18 Like ontological priority monism, Priority unity monism relies on some assumptions: (i) there is a whole, overall conscious experience that a subject has at t; (ii) the overall conscious experience has proper (experiential) parts; (iii) the overall experience is not identical to any plurality of its proper parts; (iv) there are priority relations among conscious mental states; (v) there are fundamental conscious states, in which all other states are grounded. Arguably, all proponents of SUT accept (i): if one's conscious states are unified at t, then S has a unified conscious experience at t. The pluralist holds that the unified experience depends on the conjunction of the plurality of conscious states one has at t. The priority monist maintains that the reverse priority order holds. The other assumptions, by contrast, are controversial. Before discussing possible objections to priority monism, let us briefly discuss its strengths. First, while maintaining that a subject has a number of conscious states at any one time, priority monism does not need any explanation of how they stitch together. The different conscious states a subject has at t are unified in virtue of their being proper parts of the same basic conscious experience. Their being unified needs no further explanation-this is indeed one of the main reasons for embracing a top-down rather than a bottom-up approach. Second, Priority monism has a straightforward solution to the distinguishability problem. A subject can distinguish many different conscious states at t because they are distinct, though derivative, experiential parts of the overall, basic experience she has at t. However, it seems that priority monism leaves something unexplained. According to (ii), the overall basic conscious experience one has at t has experiential parts. Just as pluralism must explain how several simultaneous conscious experiences are tied together, priority monism must account for how several conscious states, that are experiential parts of a conscious experience, hold together. As we saw, priority monism does not appeal to any special primitive relationship. They hold together in virtue of their being derivative parts of the same basic conscious experience. Hence, in order to back (ii) without incurring the difficulty faced by pluralism, priority monism resorts to (iv) and (v), namely, to the related notions of priority and grounding. But how are those notions spelled out? If an answer can be given, it is to be found in metaphysics. Thus, let us take another look at the ontological debate. 17 If the overall conscious experience is prior to the conscious states composing it, then the properties of the different conscious states depend on the properties of the overall experience. Typically, pluralists maintain that the phenomenal character of the unified conscious experience derives from the conjunction of the phenomenal characters of the different simultaneous conscious states. The overall unified conscious state has a 'global' phenomenal character, which depends on the 'local' phenomenal characters of all the simultaneous conscious states. Priority monism, by contrast, holds the reverse: it is the phenomenal character of the overall unified experience that grounds the phenomenal character of each of its experiential parts. 18 Priority unity monism does not seem to be represented in contemporary research on unity of consciousness. Perhaps Bayne and Chalmers' (2003) subsumption theory might be read as a version of Priority monism. However, there are some features of their view that seem to make it pluralistic rather than monistic. First, they write that "[a]t any given time, a subject has a multiplicity of conscious experiences." (23, emphasis mine). Second, they define subsumption as a relation: "It is a relation among token phenomenal states. It is plausibly reflexive (a state subsumes itself), antisymmetric (if A subsumes B and B subsumes A, then A = B), and transitive (if A subsumes B and B subsumes C, then A subsumes C)." (40). 9 Schaffer introduces his account of priority by saying that for an analysis of the ontological structure of the world, beside the "mereological structure of whole and part", the "metaphysical structure of prior and posterior" must be considered. The latter "reflect[s] what depends on what, and reveal[s] what are the fundamental independent entities that serve as the ground of being." (2010: 35). Priority is thus intimately related to both dependence and grounding: x grounds y only if x is prior to y; y depends on x only if x is prior to y.19 But, what is priority? Schaffer provides us with some instances of it (2010: 35). Consider Socrates and the proposition 'Socrates exists'. Straightforwardly, Socrates exists iff 'Socrates exists' is true. However, we would not be inclined to think that Socrates exists in virtue of the proposition 'Socrates exists' being true. Rather, we would say that the proposition is true because Socrates exists. Intuitively, it is being that grounds truth, rather than the other way round. Another example is the relation between Socrates and his singleton {Socrates}. In this case, it is the singleton that depends on Socrates, and not the reverse-sets depend on their members. Similarly, Schaffer argues, there are dependence relations among concrete objects. There are some concrete objects whose existence depends on others'. Thus, he continues, "it makes sense to inquire as to the dependence ordering (if any) among Socrates' snub nose, his body, and the cosmos that embodies him" (2010: 36). As noted, Schaffer characterises priority as an irreflexive, asymmetric and transitive relation. However, he does not analyse the notion of priority further, assuming that intuition provides us with a sufficient grip on it. He declares explicitly that priority is a primitive, unanalysable notion. It cannot be explained in terms of anything else. However, he argues, it is both useful and intuitively clear.20 This may appear a bit unsatisfying. After all, since Schaffer's monism entirely rotates around the notion of priority, one would expect it to be spelled out more thoroughly. Moreover, and most importantly for our purposes, priority is the key notion of Priority unity monism. It is what ultimately explains the unity of consciousness. It seems that for Priority unity monism to be viable a deeper explanation of priority is required. Thus, let us try to examine the notion more carefully. (As noted, priority, dependence and grounding are strictly connected. Thus, the examination of any of them would automatically shed light on the others.) Consider the following examples, borrowed from Schaffer (2009: 375) and Rosen (2010: 110-113): (a) The cup's brittleness depends on the arrangement of its constituent atoms. (b) The truth value of a proposition depends on how the world is. (c) Moral properties depend on natural properties. (d) Sets depend on their members. (e) The hole in the cheese depends on the cheese. As rightly pointed out by Clark and Liggins (2012), those examples suggest a first approximation characterisation of grounding. Grounding is a non-causal dependence relation, intimately related to explanation. It seems natural to think that from (a) we can infer that the molecular structure of the 19 Unless x and y are mutually dependent. 20 Cf. Schaffer (2009: 376): "So I say that grounding passes every test for being a metaphysical primitive worth positing. It is unanalysable. It is useful. And it is clear what we mean." And Schaffer (2010: 36): "Perhaps the notion of priority is amenable to further analysis [...]. In any case I think that it would be a mistake to insist that this useful and natural notion is illegitimate unless one can display its analysis. By that standard virtually no philosophical notion would count as legitimate." 10 cup explains its brittleness; from (b) that the world explains why a proposition is true rather than false; from (c) that moral features are explained by natural features, and so on. Sceptics about grounding often contend that the notion is unintelligible, given its lacking an explicit definition (Clark and Liggins 2012: 814). However, I do not believe this to be the main problem with grounding. We use the notions of grounding, priority and dependence not only in philosophy, but also in other scientific disciplines and in everyday life. Schaffer seems to be right that the three notions can be intuitively grasped. If provided with the right examples, we seem to be able to understand them. The real problem with grounding-and related notions-is that, by itself, it seems to lack explanatory power. Take the example of the cup: the cup's brittleness is grounded in its molecular structure. Though we do understand what this means, if we are provided with this statement and nothing else, we are left with little explanation of the cup's brittleness. A natural question would arise: "How does the molecular structure explain brittleness?" This question may have an answer of the kind: "The material of which the cup is made up has molecular structure S. A material with molecular structure S allows little or no plastic deformation. Thus, it breaks easily, even if subjected to little stress. This happens because the molecular bond between atoms a and b is of kind k, which is extremely weak..." Regardless of the accuracy of my account of brittleness, a reply of this kind explains the grounding relation between the brittleness of the cup and its molecular structure. It specifies why and how such a relation holds, and thereby provides it with explanatory power. Thus, we can grasp what it means for the brittleness to be grounded in the molecular structure, but only if some further information is provided. By itself, (a) says nothing about the effective relation between brittleness and atom arrangement. To be substantive, the grounding relation must be supported by some further explanation. The same holds for the other examples. Truth-value is grounded in the world because a proposition is true iff it correctly describes the world. The grounding relation between truth-value and the world is explained by the definition of truth. Moral properties depend on natural properties (according to moral naturalism) because of the effects natural facts have on human life. Sets depend on their members because sets are ensembles of things-they are determined by their members by definition. The hole depends on the cheese because there can be no hole without surrounding matter. Many other examples can be found. The temperature of a body depends on its particle kinetic energy because the more particle movement increases, the higher the body's temperature is. Phenotype depends on genotype because the phenotype is the manifestation of the genotype. And so on and so forth. Hence, any account that appeals to grounding calls for a backing explanation of why and how the grounding holds. Priority unity monism says that all the conscious states one has at t depend on the single basic conscious experience of which they are parts. Unity of consciousness is due to the fact that the overall experience is prior to its parts. However, priority monism does not seem to provide any explanation to back the priority relation between the overall experience and its parts. It only says that the overall conscious experience is prior to its experiential parts. But it does not include an explanation of why or how this priority relation holds. As long as priority is not accompanied by that explanation, appealing to grounding does not help account for the relation between the unified experience and the conscious states composing it. Hence, Priority unity monism owes us a backing explanation of grounding in the case of unity of consciousness. As long as it does not include the required explanation, it cannot be considered a satisfactory theory of unity of consciousness. My argument against priority monism may be summarised as follows: 11 P1. The priority relation provides a satisfactory account of the unity of consciousness only if there is a backing explanation of why and how priority holds. P2. Priority unity monism accounts for unity of consciousness by appeal to priority but does not provide a backing explanation of it. C. Priority monism does not provide a satisfactory account of unity of consciousness. 5. EXISTENCE UNITY MONISM Existence unity monism is the view that a subject has only one conscious experience at a time, which does not have any experiential parts. That is: EUM: All the (apparent) conscious states a subject S has at t are unified in virtue of S's having one and only one partless conscious experience at t. In spite of the appearances, the subject does not have many different conscious states at t, but one single partless overall experience. Contrary to Priority unity monism, Existence unity monism does not have a straightforward solution to the distinguishability problem. Since it claims that the overall experience does not have any parts, its solution must be more sophisticated and somewhat indirect. In order to see how it may be spelled out, let us consider first an analogous problem for Existence unity monism's ontological counterpart. The main contemporary proponents of ontological Existence monism are Terry Horgan and Matjaž Potrč (2000). According to them, there is only one concrete object, namely, the whole universe-which they call 'blobject'. The blobject has no genuine parts, though it has an extremely complex internal spatiotemporal structure. A prima facie question naturally arises. We speak of tables and chairs, planets and atoms. It seems that if Existence monism were true, all our commonsense and scientific statements about the world would be false. According to Horgan and Potrč this is not the case. Their ontological thesis is accompanied and supported by a semantic theory, according to which truth is indirect languageworld correspondence. Commonsense and scientific statements can be true, even though the nouns for objects they involve-like "table", "planet", and "atom"-actually correspond to nothing in reality. Tables, atoms and planets are not really parts of the universe, nor distinct concrete objects. They are instead conceptual posits. We directly refer to them for indirectly referring to the world. In order to clarify their theory Horgan and Potrč propose a thought experiment: "Imagine a world consisting entirely of gunkish, jello-ish, stuff. Suppose that this jelloworld is literally partless, and yet also exhibits local variation (both spatially and temporally) in features like color, transparency, density, and the like" (2000: 250). How would we describe it? We might set a conceptual framework, by individuating points or regions of the jelloworld, and attribute certain properties to them. We could thus talk about the jelloworld, though by referring to it only indirectly: we would always directly refer to the points and regions we have conceptually posited, which are not real parts of the jelloworld. The actual world, according to Horgan and Potrč, is very similar to the jelloworld. We speak of things, objects, persons, events, but they are not genuine parts of the world. They are rather linguistic or conceptual posits that, albeit indirectly, help us to tell how the world really is, and (possibly) correctly describe it. Now, let us see how this can help Existence monism about conscious unity address the distinguishability problem. The existence monist might argue that the single conscious experience 12 one has at t has a complex internal structure. What appear to be distinct conscious states are actually ways in which the unique conscious experience is structured.21 One way in which this could be spelled out is that the overall experience has a conjunctive content.22 Consider again my simultaneously seeing Mondrian's Composition, smelling roses and tasting chocolate. According to this view, my overall experience does not have a multiple content like < (I have an experience of) seeing Mondrian's Composition > and < (I have an experience of) smelling roses > and < (I have an experience of) tasting chocolate >.23 Rather, it has a conjunctive content of the kind < (I have an experience of) seeing Mondrian's Composition and smelling roses and tasting chocolate >, where the conjuncts are not parts of the overall content, but rather structural articulations of it. The conjunction reflects the way the unique experience is structured. (Of course, the conjunctive content I have just proposed is an extremely simplified report of what in reality would be a far richer and finely structured content.) Nevertheless, it seems that the mental goings-on occurring within an overall conscious experience at t are not merely ways in which it is structured. There really seem to be some kind of experiential parts we can distinguish within the whole experience. Indeed, we typically describe our conscious experience as a set of different states. For example, if asked what I am experiencing at ten to noon in the park, I might reply "I see a photograph of Mondrian's Composition, smell roses and taste chocolate", by which I plausibly mean that I have different conscious states at the same time. In general, it seems that when we speak or think about our conscious experience, what we refer to are distinct mental states, rather than a whole experience with an extremely complex content. Existence unity monism, however, claims that there are no such distinct states. It thus seems that if Existence unity monism were true, the almost all our beliefs about our own experience would be false. It might be claimed that Existence unity monism does not necessarily have this consequence. Similarly to ontological Existence monism, it may be accompanied by a special semantic theory, according to which the truth of statements about experience is indirect languageexperience correspondence. What we take to be different conscious states (my seeing the painting at t, my smelling the roses at t, my tasting chocolate at t), are indeed mere linguistic posits, which allow us to talk about our own experience. Similarly to the points and regions in Horgan and Potrč's jelloworld, experiential parts are conceptual constructs to which we directly refer in order to talk (indirectly) about our overall conscious experience. Hence, although they are not genuine parts of our conscious experience, experiential parts are useful tools through which we talk and think about its complex structure. Our statements thus refer to our experience only indirectly: we indirectly refer to our (real) conscious experience by way of directly referring to some conceptually posited experiential parts. This semantic solution, however, is open to at least one objection.24 If discourse involving experiential parts really allows one to form true judgements about one's own conscious experience, then one should be able to form judgements about one's conscious experience whose terms refer to that experience directly. Accordingly, one's indirectly true statements should be paraphrasable into 21 She might also reply that the single conscious experience, whilst lacking experiential parts, has non-experiential parts grounding its variety and richness. What these non-experiential parts would be, however, is not totally clear. 22 Cf. Tye (2003: 36-37): "[P]henomenal unity is a matter of simultaneously experienced perceptual qualities entering into the same phenomenal content [...] phenomenal unity goes with the closure of perceptual experience under conjunction with respect to the unified qualities." (italics original). 23 I put "I have an experience of" in parentheses in order to be neutral on whether one's having an experience of x is part of the content of one's experience of x or not. I am not going to take a position on this matter here. 24 See Horgan and Potrč (2000: 256) for a discussion of its ontological analogue. 13 directly true ones: one should be able to find equivalent expressions that directly refer to one's experience. However, any possible translation would involve parts of some sort that one can conceptually distinguish within her experience. Take, for example, the whole visual scene before me at ten to noon while I am sitting in the park. A natural description of it would be something like: "I see the photograph of Composition with Red, Blue and Yellow, I peripherally see the text describing the painting, and, on the background, I see a fountain". "I see the photograph of Composition with Red, Blue and Yellow" may be thus paraphrased: "At the centre of my overall visual field there is a coloured square; in its upper-right portion I see a big red square; in the bottom-left part I see a blue rectangle, and in the bottom-right corner a small yellow rectangle; at the upper-left I see a square white part and under it an identical square white part, both divided by a black rectangular part...". It seems evident that talking about visual experience necessarily involves talking about experiential parts.25 Perhaps statements about conscious states involving sense modalities other than vision might find a suitable paraphrase. "I taste chocolate", for instance, may be paraphrased into "My overall experience is gustatorily chocolated". However, a description of visual experience is impossible without 'part' expressions indicating a location in the visual field. I can say that my experience is "visually red", but I must nevertheless specify where. The same is true for bodily sensations: I can say that my experience is painful, but I have nonetheless to specify the part of the body where I feel pain. Hence, it might be that a part of the overall experience (the one concerning sense modalities other than vision) can be described directly-provided that we find "gustatorily chocholated" a satisfying description of my experience. However, visual and proprioceptive state descriptions cannot be paraphrased into expressions which directly refer to the structure of the overall experience. Thus, not all the indirect statements about the overall conscious experience can be paraphrased into direct ones. My argument against Existence unity monism is thus the following: P1. If indirect language-experience correspondence allows us to form true judgements about our own conscious experience, then all indirectly true statements about our experience must be paraphrasable into directly true ones. P2. Not all indirectly true judgements about our conscious experience are paraphrasable into directly true ones. C. Indirect language-experience correspondence does not allow us to form true judgements about our own conscious experience. Although it might occur that we form false introspective beliefs, few would agree that our judgements about our conscious experience are always false. A theory which implies that we cannot think truly about our experience should arguably be rejected. In support of P2 is the fact that, as shown, there are at least two kinds of introspective judgements (visual and proprioceptive) for which a suitable existence-monist paraphrase cannot be found. What the existence monist can do to avoid C is denying P1, and argue that the paraphrase of indirectly true statements in directly true ones is not necessary for holding true beliefs about one's 25 We might also set a two-dimensional coordinate system, and describe our visual field by assigning a colour to each point individuated by a pair of coordinates. So, for example, I might describe my experience of the painting by saying "blue in (1, 1), blue in (1, 2), ... yellow in (30, 1), ... red in (30, 30), etc." However, points individuated by coordinate pairs would still be distinguishable parts of my visual experience. 14 conscious experience. To see how this is supposed to work, let us go back to ontological Existence monism. The analogous objection to ontological Existence monism is that if ordinary statements about the world (like "there is a laptop before me") are not paraphrasable into statements whose terms directly refer to the blobject (statements that describe it as it really is, so to speak), then it is impossible for us to form any true judgement about the world. Paraphrasing statements containing indirectly referring expressions into statements containing only directly referring ones would involve finding "systematic, tractably specificable, truth conditions for posit-employing discourse [that] could be formulated in language that eschews all such posits and talks only about the blobject and its attributes." (Horgan and Potrč 2000: 256). Since nothing can be said about the blobject by employing directly referring expressions, such truth conditions cannot be given. Horgan and Potrč acknowledge that. However, they argue, this does not entail that all our judgements about the world are false: "given the general conception of truth as semantic correctness under contextually operative semantic standards, there simply need not be (and very probably are not) those kinds of truth conditions" (ibid.). On the semantic standards that apply to the context of discourse about the world, statements involving conceptual constructs are (possibly) semantically correct, even though the posits corresponding to such constructs do not exist in reality. Hence, the truth of statements about the world, on their view, simply is indirect language-world correspondence. Truth being thus defined, our statements about the world can be, and indeed often are, according to Horgan and Potrč, genuinely true. By transposing Horgan and Potrč's argument to the context of unity of consciousness, we have that statements mentioning experiential parts (or conscious states) can be semantically correct even though in fact there are no such experiential parts. In the context of the discourse about conscious experience, truth simply is thus indirect language-experience correspondence. Given this new definition of truth, our statements involving reference to experiential parts are (possibly) genuinely true. In sum, Existence monism's strategy consists in denying P1 by formulating an ad hoc definition of truth which allows statements about conscious experience to be possibly true even though the posits to which their terms refer (conscious states, or experiential parts) do not really exist. However, this is somewhat unsatisfactory. Introducing a novel, ad hoc story about truth is not a neutral move. Rather, it seems to be a cost for Existence unity monism. Arguably, it is a merit of a theory that it achieves the maximal explanatory power with the least theoretical upheaval. Correspondingly, introducing unconventional ad hoc explanations is often a cost. To be sure, this is a price it is sometimes worth paying, if the theory's benefits not only balance but overweigh the costs. But if the same explanatory power can be achieved without any ad hoc move, then the theory is better abandoned. For, plausibly, if two theories are explanatorily equivalent, then the more conservative must be preferred. In the next section I will present Brentano's theory of unity of consciousness. Brentano's theory has the benefits of Existence unity monism, but does not introduce any ad hoc story of truth. Moreover, from a consideration of Brentano's mereological ideas a further objection against Existence unity monism will emerge. 6. BRENTANIAN UNITY MONISM Brentano formulates his view about the unity of consciousness as follows: 15 The unity of consciousness, as we know with evidence through inner perception, consists in the fact that all mental phenomena which occur within us simultaneously such as seeing and hearing, thinking, judging and reasoning, loving and hating, desiring and shunning, etc., no matter how different they may be, all belong to one unitary reality only if they are perceived as existing together. They constitute phenomenal parts of a mental phenomenon, the elements of which are neither distinct things nor parts of distinct things but belong to a real unity. (1874/1973: 163-64) Brentano defends his theory against both those who deny that human consciousness is unified, and those who maintain a pluralistic view. Among the latter is David Hume, whose theory is that at any one time our consciousness is constituted by a bundle of different ideas somehow tied together. Brentano outright rejects Hume's theory: A 'bundle', strictly speaking, requires a rope or wire or something else binding it together. In the case of human consciousness it is out of question that there is something of this sort, or even just something analogous to it. (1982/1995: 14) Brentano maintains that at any one time consciousness, rather than a multitude of ideas, is a unified single thing.26 He is thus clearly a monist about unity of consciousness. Interestingly, he is well aware of the natural objection against any monistic view, namely what I called the distinguishability problem. And indeed he promptly points out that Our consciousness does not present itself to our inner perception as something simple, but it shows itself as being composed of many parts. Unity of reality is something different from simplicity of reality. (1982/1995: 15) Given his argument against Hume, by 'parts' Brentano plausibly does not mean distinct conscious experiences or separate ideas. Rather, what he seems to have in mind is a special sort of experiential parts, such that, whilst they can be distinguished from one another, they nonetheless necessarily belong to one and the same conscious experience. It might be objected that there are some passages in his writings which suggest this to be an inaccurate interpretation of Brentano's. For example, in Descriptive Psychology he seems to claim that, although some conscious states are not separable from one another, others are: "[O]ur consciousness is composite and it allows us to distinguish parts, some of which can be actually separated from other ones" (15). He explains how some conscious states may be separated: The sense in which one of these parts can be actually separated from one another is that the former, having existed earlier as belonging to the same real unit as the latter, continues to exist when the latter has ceased to be. (1982/1995: 15). However, one possible interpretation of this passage is the following. That two conscious states can be actually separated from one another might just mean that, while being necessarily unified at t, they may stop being consciously undergone together at a later time t1 (because one of them has ceased to exist). This does not entail that at t the two states are actually separated. On the contrary, since at t they necessarily belong to the same unity, they are inseparable though distinguishable parts of one single experience. It might turn out that this interpretation is inaccurate, and that Brentano really maintained that some conscious states of a subject at a time can be separated from the others. If it is the case, then he might have missed out on a very plausible view-indeed, as I am going to argue, the most plausible monistic view. What seems to be true, in any case, is that he maintained that at least some 26 "No multitude of things, but most unambiguously a single thing, embracing the whole of an actual human consciousness." (Brentano 1982/1995: 14). 16 parts of one's conscious experience at a time are not separable from others. They are parts of a very special kind, which can be distinguished from one another, although they necessarily belong to the overall experience. To see what kind of parts Brentano has in mind, we must look at his mereological ideas, and more specifically at the notions of distinctional and separable parts. Separable parts are "parts which are actually separable from one another" (Brentano 1982/1995: 16). X and Y are separable parts of a whole W iff they can exist independently of W: they can be separated from W and from each other. W is, in this case, a collection of things, rather than a really unitary thing. X and Y are tied together, and so are parts of W. Nevertheless they are not necessarily bound. X and Y belong to W only contingently: the existence of either depends neither on W nor on the existence of the other. For instance, pages are separable parts of a book. They are bound together, but they can continue to exist even if separated from the book and from each other. Distinctional parts are "parts which are distinguishable even though they are not actually separable" (ibid.). X and Y are distinctional parts of a whole W iff they cannot be separated from W nor from each other. They can be only conceptually discerned. One can single them out from the whole, and distinguish them from one another, but cannot take them apart. In this case, W is not a collective of things, but a full-fledged unitary thing. Its parts necessarily belong to it: the existence of X and Y depends on the existence of W. As a clarification, Brentano suggests the following example. Consider mereological atoms.27 Even though separating any part of an atom is impossible (by definition), we can still discern the atom's parts in thought. For instance, we can discern its upper and bottom halves: we can divide it in two in thought, and distinguish one part from the other. The upper and the bottom halves are distinctional parts of the atom. On the view I want to defend, what are typically considered the conscious states one has at any one time are distinctional parts of one single conscious experience. I call this view Brentanian Monism: BUM: All the (apparently distinct) conscious states a subject S has at t are unified in virtue of S's having one single conscious experience at t, of which all the (apparently distinct) conscious states are distinctional parts. As noted, this might not be Brentano's own theory of the unity of consciousness. It is nevertheless Brentanian for two reasons. First, the notion of distinctional part belongs to Brentano's mereology. Second, it is Brentano's view that at least some of the conscious states a subject has at a time are distinctional parts of her overall experience. Within this framework, pluralism is the view that at t S has a collection of separate conscious experiences. On Brentanian monism the overall unified experience is not a collection of separate conscious states. It is one single state of consciousness. What we take to be different conscious states are actually distinctional parts of the single overall experience. As noted, distinctional parts ontologically depend on the whole to which they belong-they cannot exist separately from it. If we translate this in terms of priority, we have that if X and Y are distinctional parts of W, then W is ontologically prior to X and Y.28 Hence, on Brentanian monism, all the conscious states one has at t ontologically depend on the overall experience of which they are 27 Contrary to physical atoms (which have subatomic particles as parts) mereological atoms are partless by definition. 28 For a discussion of separable and distinctional parts, and their relationship with the notions of dependence, priority and grounding, see Kriegel (forthcoming). 17 distinctional parts. They cannot be experienced separately from it. Therefore, the overall experience is prior to its experiential parts. Interestingly, the resulting account looks very similar to Priority unity monism. We saw that the main problem with Priority monism is that it explains unity in terms of priority without providing the latter with a suitable backing explanation. On Brentanian monism as well there is a relation of priority between the overall experience and its parts. However, this is not supposed to explain unity. Rather, it is a constitutive fact of the way consciousness is unified. Moreover, priority is not left unexplained on Brentanian monism. The priority relation between the overall experience and its experiential parts holds in virtue of the latter's being distinctional parts of the former. Thus, Brentanian monism provides priority with a backing explanation, and does not incur the difficulty met by Priority monism. Brentanian monism also overcomes the difficulties met by Existence unity monism. Recall, Existence monism introduces an ad hoc story about truth, according to which we can never think or talk about our experience directly, but always indirectly, via referring to some experiential parts which do not exist in reality. According to Brentanian monism, instead, the conscious states to which we directly refer when we think about our conscious experience do exist in reality. They exist as distinctional parts of the overall experience to which they belong. Accordingly, they are fully-fledged existing entities, although they cannot exist independently of the overall experience. When we describe our conscious experience by way of statements involving conscious states, we directly refer to existing entities. Moreover, since conscious states are distinctional parts of one and the same experience, by referring to the conscious states we thereby directly refer to the overall experience. Therefore, Brentanian monism does not need to be supported by any ad hoc semantic theory. Since its explanatory power is at least equal to Existence monism's, and since it is more conservative, Brentanian monism seems to be preferable. Moreover, as anticipated at the end of section 5, Existence unity monism faces a further difficulty, as made clear by reflecting on Brentano's notion of distinctional part. Existence monism denies the existence of any genuine experiential parts. Nevertheless, talk in terms of experiential parts is not only allowed, but also necessary-it is the only way to talk about our conscious experience at t. Now, it seems that if we can talk about experiential parts, then, a fortiori, we must somehow distinguish them-conceptually single them out from the overall experience. But this entails that Existence monism must allow for distinctional experiential parts. Now, arguably, distinctional parts are fully-fledged (genuine) parts. Consider again Mondrian's Composition with Red, Blue and Yellow (consider the painting now, not my visual experience of it). I can distinguish a blue rectangle in it, at the bottom-left, even though the rectangle cannot be separated from the painting. (We might want to tear it up. However, we would not separate a part of the painting, but rather a part of the canvas, and bring the painting out of existence). The painting really has a rectangular blue part at the bottom-left corner. This is a genuine part of the painting, though it can only be distinguished, but not separated. Similarly, at ten to noon I can distinguish my seeing the painting from my overall experience. My seeing the painting is thus a genuine (distinctional) part of the overall experience. Therefore, it seems that Existence monism must admit that one's overall experience at t has at least one kind of genuine parts, namely distinctional parts. Denying distinctional experiential parts would imply denying any possibility of talking about one's overall experience. If so, it seems that Existence monism has much in common with Brentanian monism. However, as noted, since Brentanian monism does not need any ad hoc story about truth, it should be preferred to Existence monism. 18 Brentanian monism, therefore, avoids the problems affecting both Priority and Existence unity monism. Nonetheless, it preserves their virtues. It claims, with Existence monism, that a subject has one single experience at t. Thus, it does not face the difficulty of explaining how different conscious experiences relate to each other. Furthermore, it maintains, with Priority monism, that the whole conscious experience has experiential parts (although these experiential parts don't get to count as themselves conscious experiences), thus accounting for our ability to discern different goings-on within the same experience. It is clear that Brentanian monism elegantly solves the distinguishability problem. A subject can discern a multiplicity of goings-on within the unique conscious experience she has at t because they are distinctional parts of the same conscious experience-parts that can be distinguished, though not separated from it. Moreover, Brentanian monism provides a possible explanation the fact that, necessarily, all one's conscious states at t are unified (as per SUP). As we saw, distinctional parts necessarily belong to the whole which encompasses them: they cannot be separated from the whole and their existence depends on it. Hence, all conscious states a subject has at a time are necessarily unified in virtue of their being merely distinctional parts of one single conscious experience at t. Given that Brentanian monism overcomes the difficulties met by Existence and Priority monism, while maintaining their strengths, I conclude that it is the best top-down approach to unity of consciousness. CONCLUSION I have presented three possible monistic theories of unity of consciousness. The main challenge all of them face is what I called the distinguishability problem: how can we discern different conscious states (as we do), if at any one time we have one single experience? Priority monism replies that we do not quite have one single experience, but rather one single basic experience, of which the different conscious states we distinguish are derivative experiential parts. As I showed, this response is not satisfying without a backing explanation of the priority relation among conscious states. Existence monism claims that what we take to be different conscious states are actually ways in which the one single partless experience we have at t is complexly structured. The theory is supported by an indirect language-experience correspondence account of truth: we conceptually posit experiential parts for indirectly referring to our really partless experience. However, this story about truth is an avoidable ad hoc move. I concluded that Brentanian monism is the best top-down approach to unity of consciousness. By claiming that all the conscious states a subject has at t are distinctional parts of one and the same conscious experience, it preserves the strengths of both Priority and Existence monism and avoids their difficulties. Moreover, it elegantly solves the distinguishability problem, and provides a potential explanation of the unity thesis. Thus, Brentanian monism is the best topdown approach to unity of consciousness, and a good candidate to challenge bottom-up approaches.29 29 I am extremely grateful to Uriah Kriegel for his comments on previous drafts of this paper, and for the long discussions we had about the unity of consciousness. Without his help this paper would not exist. I am also grateful to 19 REFERENCES Bayne, Tim. 2010. The Unity of Consciousness. Oxford: Oxford University Press. Bayne, Tim, and David J. Chalmers. 2003. 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