Perception, Cognition and Aesthetics (ed. by Dena Shottenkirk, Manuel Curado and Steven S. Gouveia) Michelle Liu Phenomenal Experience and the Thesis of Revelation Abstract: In the philosophy of mind, revelation is the claim that the nature of qualia is revealed in phenomenal experience. In the literature, revelation is often thought of as intuitive but in tension with physicalism. While mentions of revelation are frequent, there is room for further discussion of how precisely to formulate the thesis of revelation and what it exactly amounts to. Drawing on the work of David Lewis, this paper provides a detailed discussion on how the thesis of revelation, as well as its incompatibility with physicalism, is to be understood. Keywords: revelation, qualia, phenomenal experience, David Lewis, physicalism. 1. Introduction The term "revelation," introduced by Mark Johnston (1992), is used in the debate about the metaphysics of color properties to refer to the view that the whole nature of color properties is given in color experience. This paper is not concerned with the thesis of revelation in the philosophy of color, but with a parallel idea that also goes by the name "revelation" in the philosophy of mind. This is the claim that the whole nature of qualia (phenomenal properties) is given in phenomenal experience. This latter thesis of revelation is by no means unfamiliar to philosophers of mind, although the name "revelation" has not always been used to identify it. Discussions on the idea of revelation can be traced to Descartes (1985: 215-7) in his writings on the notion of a clear and distinct idea, and Hume (2000: 136) in the Treatise.1 One can also find contemporary discussions on revelation in relation to the topic of phenomenal experience and the physicalist-dualist debate about the nature of consciousness (Strawson, 1989; Lewis, 1995; Nida-Rümelin, 2007; Stoljar, 2006, 2009; Damnjanovic, 2012; Goff, 2015, 2017; Majeed, 2017; Trogdon, 2017). Two things are worth noting regarding the contemporary literature on revelation. First, revelation is often thought of as being in tension with physicalism. For instance, Lewis (1995) takes revelation to be incompatible with physicalism. More recently, Philip Goff (2015, 2017) has appealed to the thesis of revelation to argue against physicalism. Of course, whether there is an incompatibility between revelation and physicalism, and how this incompatibility is to be understood provided that there is one, depends on how we cash out revelation and physicalism (see Damnjanovic, 2012; Trogdon, 2017; Stoljar, 2018). The second thing worth noting about the contemporary literature on revelation is that revelation is often thought of as an intuitive claim. Among those who think this way about revelation are physicalists who also acknowledge the incompatibility between revelation and physicalism (Lewis, 1995: 142; McLaughlin, 2003: 378; Braddon-Mitchell, 2007: 287; Hill, 2014: 199-200). For instance, Lewis (1995) and Braddon-Mitchell (2007) take revelation to be intuitive because they think that it is part of the ordinary conception of experience. Despite thinking that it has an intuitive appeal, these physicalists nevertheless deny the thesis of revelation in order to safeguard physicalism. Of course, the aforementioned claims about revelation, namely, that it serves as a crucial premise in an argument against physicalism and that it is intuitive, are points of contention in the literature. Regarding the former, one might dispute the force of the argument from revelation against physicalism. For instance, one might query whether the argument proves that all versions of physicalism are false; one might also argue that revelation is in fact implausible (see Damnjanovic, 2012; Trogdon, 2017; Stoljar, 2018). With respect to the intuitiveness of revelation, one might question whether revelation is in fact intuitive or part of the ordinary conception of experience as is often claimed (see Stoljar, 2009). In order to gain a better understanding of these debates about revelation we need, first and foremost, to clarify the idea of revelation itself. While there are frequent mentions of revelation in the philosophy of mind, as we have just seen, there is room for further discussion of what the thesis of revelation precisely amounts to. The starting point of the thesis is the claim that by having a phenomenal experience, one is in a position to introspect that experience and to form certain thoughts – make certain judgements, achieve certain knowledge – which are "revelatory" about the experience. The sense in which such thoughts are "revelatory" requires careful articulation. According to Lewis (1995), the kind of thought about phenomenal experience under consideration amounts to knowing the essence of the qualia of a given experience. A main goal of this paper is to look into Lewis' remarks in detail and clarify what these revelatory introspective thoughts about phenomenal experience are supposed to be. According to the formulation presented in this paper, revelation is a thesis about qualia, which are the phenomenal properties of our experience. The thesis claims that by having an experience-token with a particular quale Q, one is in a position to form a certain thought or know a certain truth, namely, "Q is X," where the predicate "X" captures the essence of Q, although it may be hard to put into words. Having clearly articulated the thesis of revelation, I turn to explain how the thesis of revelation thus understood is incompatible with versions of physicalism. Overall, this paper aims to make important clarifications that shed light on existing debates regarding revelation. The structure of the paper is as follows. In section 2, I examine the thesis of revelation as presented in Lewis' 1995 paper "Should a Materialist Believe in Qualia?." Lewis' paper is particularly important to the literature on revelation because ideas in this paper have given rise to much discussion on the topic (see Stoljar, 2006, 2009; Majeed, 2017). Section 3 goes beyond Lewis' remarks on revelation and puts forward a more precise formulation of the thesis of revelation. Sections 4 and 5 make two further clarifications regarding the thesis. In section 4, I make clear that the thesis of revelation involves a knowledge-attribution de dicto regarding knowledge de dicto that a particular quale Q is X, where the predicate "X" captures the essence of Q. In section 5, I clarify how the thesis of revelation might plausibly entail the claim that qualia are simple. Section 6 explains how revelation thus understood is incompatible with certain versions of physicalism. Section 7 concludes the paper. 2. Lewis on Revelation In his article "Should a Materialist Believe in Qualia?" Lewis discusses the thesis of revelation, which he calls "the Identification Thesis." Lewis' account of revelation is framed in terms of the notion of qualia. The term "qualia" in this context refers to the phenomenal character or the what-it-is-likeness of our experience. Qualia are phenomenal properties of particular events of experiencing. Different types of phenomenal experience are individuated by their qualia. Token experiences with the same qualia belong to the same type of experience.2 Although "quale" and its plural, "qualia," are technical terms, the concept, as Lewis sees it, is nevertheless part of our folk psychology. As a functionalist, he (1995: 140) regards the term "qualia" as "a name for the occupants of a certain functional role that is spelled out in our tacitly known folk psychology." According to Lewis, the folk-psychological role of qualia includes at least the following four theses: (i) Qualia cause our "abilities to recognize and to imagine experiences of the same type" (Lewis, ibid.: 141); (ii) Qualia are "responsible for responses of pleasure, disgust, etc." (op. cit.); (iii) Qualia are "responsible for judgements of similarity-distance, e.g., the judgement that these two colour-samples nearly match whereas those two contrast strikingly" (op. cit.); and, (iv) "We identify the qualia of our experiences." (op. cit.) While the first three components of the qualia-role are clear, the fourth one requires elaboration, especially with respect to the notion of identification. Lewis elaborates on this component of the qualia-role: Folk psychology says, I think, that we identify the qualia of our experiences. We know exactly what they are – and that in an uncommonly demanding and literal sense of 'knowing what'. If I have an experience with quale Q, I know that I am having an experience with quale Q, and I will afterwards remember (unless I happen to forget) that on that occasion I had an experience with quale Q. It is by producing this identifying knowledge that a novel experience confers abilities to recognize and imagine. (op. cit.) Consider the experience of tasting the Australian spread Vegemite. In having such an experience, one might have various thoughts about the experience and come to know numerous things about it, e.g. "This experience is interesting," "I have had this experience before," "It is similar to the experience of tasting the British spread Marmite," etc. Among these thoughts is, Lewis would say, the following one: "I (now) know exactly what the phenomenal character of the experience of tasting Vegemite is." This latter knowledge is an instance of what Lewis (op. cit.) calls "the Identification Thesis" – "We know exactly what [qualia] are." According to Lewis' understanding of folk psychology, knowing exactly what the quale of the experience is confers one's abilities to remember, recognize and imagine experiences of the same type. But what does Lewis mean when he says that "we know exactly what [qualia] are"? Lewis clarifies: I spoke of 'an uncommonly demanding and literal sense of "knowing what"'. Let me elaborate. I say that according to the Identification Thesis, the knowledge I gain by having an experience with quale Q enables me to know what Q is – identifies Q – in this sense: any possibility not ruled out by the content of my knowledge is one in which it is Q, and not any other property instead, that is the quale of my experience. Equivalently, when I have an experience with quale Q, the knowledge I thereby gain reveals the essence of Q: a property of Q such that, necessarily, Q has it and nothing else does. (Lewis, ibid.: 142) According to Lewis, by having an experience with a particular quale, we are in a position to know exactly what that quale is in the sense that we are in a position to know the essence of the quale. This is what Lewis calls "an uncommonly demanding and literal sense of 'knowing what'." This demanding sense of "knowing what a quale is" contrasts with a less demanding sense of "knowing what" (Lewis, ibid.: 143). In everyday contexts, we can say that one knows what something is even if one does not know the essence of that thing. One knows what water is in an everyday non-scientific context insofar as one knows a cluster of descriptions about water, and knows how to identify ordinary samples of water, etc. To know what water is in this less demanding everyday sense of "knowing what" does not require one to know the essence of water, i.e. know that water is H2O. 3. The Formulation What Lewis calls "the Identification Thesis" is just the thesis of revelation (see Stoljar, 2006, 2009). As we have just seen, Lewis formulates the thesis of revelation as the following: [W]hen I have an experience-token with quale Q, the knowledge I thereby gain reveals the essence of Q: a property of Q such that, necessarily, Q has it and nothing else does. (Lewis, 1995: 142) There are two things to notice in Lewis' formulation of the thesis of revelation. First, Lewis is adopting a modal account of essential properties and of essence, according to which an essential property of a thing is just a necessary property of that thing, and the essence of a thing is a property that is both necessary and sufficient for being that thing. One might not want to commit to this simple modal account of essence given its wellknown problems, discussed by Kit Fine (1994). Fine (ibid.: 4-5) provides several counterexamples to the modal account. For instance, Socrates has the necessary property of belonging to the singleton set that has Socrates as its only member, i.e. {Socrates}. But intuitively, it is not part of the essence of Socrates, that is, it is not an essential property of Socrates, that he belongs to the singleton set {Socrates}. Fine, for his part, opts for the real definitional account of essence, which can be traced back to Aristotle and characterizes the notion of essence on the model of definition. According to this account, "the notion of definition has application to both words and objects – ... just as we may define a word, or say what it means, so we may define an object, or say what it is" (Fine, ibid.: 2). Here, in cashing out the thesis of revelation, we can simply take the notion of essence in the intuitive Aristotelian/Finean sense, conceived on the model of definition.3 The second thing to notice in Lewis' formulation of the thesis of revelation is that, on the face of it, the essence of Q, according to Lewis, is a second-order property. The notion of qualia here is equivalent to the notion of phenomenal qualities or phenomenal properties. The essence of Q is then a property of a property. To know exactly what Q is, for Lewis, is to know that phenomenal property Q has the (second-order) property X where X is the essence of Q (that is, Q has X and nothing else does). Without resorting to talk of second-order properties, I propose to understand "knowing exactly what a quale is," i.e. knowing the essence of a quale, as knowing some truth.4 (This makes the task of formulating the thesis of revelation simpler, and is in fact what Lewis goes on to say.5) We can say that one knows exactly what P is if and only if one knows that "P is thus and so" where "thus and so" defines P, that is, captures the essence of P. For instance, one knows exactly what the property being triangular is if and only if one knows a truth that captures what the property (by definition and essentially) is, e.g. "Being triangular is having a three-sided closed shape." The whole proposition is a definition of the property being triangular, stating what it is to be triangular. The predicate "having a three-sided closed shape" captures the essence of triangularity. In the same way, knowing exactly what quale Q is amounts to knowing the truth "Q is X," where "X" is a predicate that captures Q's essence. The thesis of revelation says that one is in a position to know this truth about Q merely on the basis of having an experience with quale Q. However, in the case of a quale, if the thesis of revelation is true, one might think that the truth "Q is X" is hard to put into words. It seems that with many experiences, we find ourselves devoid of words to describe the qualia or the phenomenal characters of these experiences. Let us call the characteristic quale (phenomenal property) of experiences of red things, "phenomenal redness." If, as I have or recall, an experience with that quale, I attempt to define what the quale phenomenal redness is, it might seem that all I am able to say is that "It is that," using a demonstrative to refer to the phenomenal character of the phenomenal redness experience. But intuitively, the demonstrative "that" is merely a placeholder for the rich understanding of phenomenal redness I have which I am unable to put into words. The fact that I cannot put my understanding into words does not mean that I don't know exactly what phenomenal redness is. Given our discussion, we can sharpen Lewis' idea of revelation and formulate the thesis in the following way: (Revelation) By having an experience-token with phenomenal property Q, one is in a position to know a truth, namely, "Q is X," where the predicate "X" captures the essence of Q, although it may be hard to put into words.6 Several things are worth pointing out with respect to the above formulation of revelation. First, the knowledge we have with respect to quale Q is a kind of knowledge-that or knowledge de dicto. The dictum is "Q is X." More needs to be said about this knowledge de dicto. I will discuss this in the next section. Second, the knowledge de dicto "Q is X" is not just any truth about Q; it is a statement about what makes Q the property it is – the predicate "X" captures the essence of Q. More needs to be said about the notion of "capture" at issue. I will address this in section 5 when discussing Lewis' argument from the thesis of revelation to the simplicity of qualia. Third, the relevant knowledge about the essence of Q is obtained under a certain circumstance, namely, by having the experience. The knowledge at issue is made available in virtue of undergoing the experience alone. Fourth, the thesis of revelation does not imply that whenever one undergoes an experience with quale Q, one automatically knows the essence of Q; it only implies that one is in a position to know the essence given that one is not distracted, is able to attend to the experience, etc. 4. Knowledge De Dicto versus Knowledge De Re In the last section, we have seen that the thesis of revelation is about a kind of knowledge de dicto. This point is worth dwelling on. In this section, I shall clarify that the thesis of revelation is best understood as involving a knowledge-attribution de dicto regarding knowledge de dicto that a particular quale Q is X, where the predicate "X" captures the essence of Q. To aid our discussion, I appeal to the following two distinctions (see Broackes, 1986): (1) knowledge de dicto vs. knowledge de re (2) knowledge-attribution de dicto vs. knowledge-attribution de re7 (1) is a distinction between different kinds of knowledge. Knowledge de dicto consists in a relation between the knower and a certain proposition, whereas knowledge de re consists in a relation between the knower and a thing (see Sosa, 1970: 883).8 (2) is a distinction between different types of reports of knowledge. Knowledge-attributions de re, but not knowledge-attributions de dicto, permit substitution of co-designating terms salva veritate (i.e. preserving the truth-value of the proposition). Given the two distinctions, one must not assume that knowledge de dicto is always reported with knowledge-attributions de dicto, or knowledge de re reported with knowledge-attributions de re. Which kind of knowledge-attribution to use often depends on the context in which the knowledge report is made, to whom the report is made, the purpose of the report, etc. If a thinker thinks about something, she thinks about it under some mode of presentation, e.g. thinks of Venus as Hesperus, or as Phosphorus. But a report of the thinker's thinking might not specify that mode of presentation. In general, a knowledge-attribution de dicto specifies the mode of presentation of the subject's knowledge, whereas a knowledge-attribution de re does not. As such, if we are interested in the mode of presentation, we should report the knowledge at issue with an attribution de dicto. For instance, suppose Tom thinks of Hesperus as Hesperus and has knowledge de dicto that Hesperus appears in the evening sky. Suppose also that Tom does not think of Hesperus as Venus. In such a case, it would still be correct to report Tom's knowledge in the following way: "Tom knows, of Venus, that it appears in the evening sky."9 But this attribution de re does not specify the way Tom thinks about Venus and consequently is silent about whether or not Tom knows de dicto that Hesperus appears in the evening sky. It seems that Tom's knowledge is best reported in the de dicto way – "best reported" in the sense that the report is as accurate and complete a guide as possible with respect to the knowledge at issue, with respect to the way in which Tom thinks about Venus (Broackes, 1986: 375). With the two distinctions above, let us return to the topic of revelation. The thesis of revelation says that by having an experience with quale Q, one is in a position to know a truth or a dictum, namely, "Q is X," where the predicate "X" captures the essence of Q. It seems that this knowledge de dicto is accurately reported by a knowledge-attribution de dicto as below: (D) By having an experience with quale Q, S is in a position to know that Q is X, where the predicate "X" captures the essence of Q. To report it with a knowledge-attribution de re fails to completely capture the kind of knowledge at issue. A knowledge-attribution de re in this case looks something like this: (R) By having an experience with quale Q, S is in a position to know of X, where the predicate "X" captures the essence of Q, that Q is it. Compare (R) with the de re report, "Tom knows of Venus that Hesperus is it." Suppose Tom thinks of Venus as Phosphorus and knows de dicto that Hesperus is Phosphorus. In such a case, the de re report is true. But this does not imply that Tom knows de dicto that Hesperus is Venus. The de re report is compatible with Tom having no idea what Venus is. Equally, (R), while stating that S is in a position to know de re of X, that Q is it (where the predicate "X" captures the essence of quale Q), does not imply that S is in a position to know de dicto that Q is X (where the predicate "X" captures the essence of quale Q). In general, one can have knowledge de re of the essence, E, of something, Φ, without knowing de dicto that Φ is E. Let's agree that water is H2O, where the expression "H2O" captures the essence of water. Someone who has no knowledge of chemistry may still know de re of H2O – thought of as water – that water is it, although she does not know de dicto that water is H2O. Similarly, there could be cases of knowledge de re of X that Q is it without knowledge de dicto that Q is X. In such cases of knowledge de re, X is thought of under some mode of presentation but not as X. So, (R) is compatible, in a perfectly good sense, with S having no idea of what X is, and with S having no knowledge de dicto that Q is X. In section 6 where we cash out the incompatibility between revelation and physicalism, we will see that for a physicalist, "X," which captures the essence of Q, is a physical/functional predicate, and while a physicalist can endorse (R), she cannot endorse (D). To sum up, revelation is not the claim that experience merely puts one in a position to know de re of X, under some mode of presentation – not necessarily as X – that Q is it (where the predicate "X" captures the essence of quale Q) (see Stoljar, 2009). Rather, revelation is the claim that experience puts one in a position to know de dicto that Q is X (where the predicate "X" captures the essence of Q). This knowledge de dicto is best reported with a knowledge-attribution de dicto in the manner of (D). The knowledge de dicto that "Q is X" may not be such that it can readily be expressed in words, but it does involve a rich understanding of X, which is what Q essentially is. 5. Revelation and the Simplicity of Qualia Another notion that is worth pausing on is the notion of capture in the formulation of the thesis of revelation which I have presented at the end of section 3. Recall the formulation: (Revelation) By having an experience-token with phenomenal property Q, one is in a position to know a truth, namely, "Q is X," where the predicate "X" captures the essence of Q, although it may be hard to put into words. The word "capture" has a number of meanings. "Capture" here should not be taken just to mean "refer to, latch onto." It is meant to convey the idea of "representing accurately in words or pictures" as in "A vivid photograph captures the scene."10 For instance, the truth "Being triangular is having a three-sided closed shape" captures or describes accurately what the property of being triangular is. Note that this truth describes the internal structure of the property of triangularity – having three sides and being closed. If a property P has an internal structure, assuming that a property's structure is essential to that property, then a truth that captures the essence of property P must also describe the internal structure of P. If a truth that captures the essence of P does not describe P as having an internal structure, then P does not have an internal structure, i.e. it is simple. Recall that in our formulation of the thesis of revelation, the truth "Q is X" is supposed to describe Q's essence. In cashing out revelation, we agreed that "X" might be difficult (even impossible) to put into words. In cases where Q seems indescribable, we might just say "this" (pointing inwardly to an experience of Q we have in mind). The intuition that at least in some cases we cannot say more about Q's essence than "Q is this" arises from the fact that in these cases, in having an experience with quale Q, we do not seem to notice Q to have any structure. Given revelation and assuming again that a property's structure is essential to that property, if quale Q had internal structure, then by having an experience with Q, one would be in a position to know that Q had that structure. The description of that structure would be given in stating that Q is X, where the predicate "X" captures the essence of quale Q.11 If there is no structure to notice with respect to Q, then Q has no structure, i.e. Q is simple. Indeed, in the following passage, Lewis remarks on the simplicity of qualia, which he takes to follow from the thesis of revelation: If we know exactly what the qualia of our experiences are, they can have no essential hidden structure – no 'grain' – of which we remain ignorant. (If we didn't know whether their hidden 'grain' ran this way or that, we wouldn't know exactly what they were. Whatever we might know about them, we would not fully know their essence.) But if nothing essential about the qualia is hidden, then if they seem simple, they are simple. We may assume that if a property is structural, then it is so essentially. Then it is a consequence of the Identification Thesis that if we fail to notice structure, there is no structure there to notice. But we do fail to notice structure. So the simplicity of the qualia is a consequence of the Identification Thesis (inter alia), and so a derivative part of the folkpsychological concept of qualia. (Lewis, 1995: 142) The above passage can be formulated into the following argument: (a) Structure is part of the essence of a property. (b) By having an experience-token with phenomenal property Q, one is in a position to know a truth, namely, "Q is X," where the predicate "X" captures the essence of Q. (c) If Q has internal structure, then by having an experience with Q, one would be in a position to know that Q has internal structure. (d) There is at least a range of cases such that in having an experience with Q, one is not in a position to know that Q has internal structure. (e) Some qualia have no internal structure (i.e. they are simple). Premise (a) seems intuitive. If a property has a certain structure, it has it essentially – a property lacking that structure would not be the same property. (b) is the thesis of revelation. (c) follows from (a) and (b). (d) seems intuitively true – we just do not notice any structure in having experiences such as phenomenal redness, hearing middle C, etc. (Some other qualia do seem to have structural features, e.g. the quale of a certain complex visual experience.) Conclusion (e) then follows from (c) and (d). So for Lewis, the thesis of revelation also entails that (some) qualia are simple (see also Adams, 1987). 6. Revelation and Physicalism Having clarified the thesis of revelation, in this section I turn to address the primary significance of revelation in the literature, namely, its incompatibility with physicalism. As already mentioned in section 1, how the incompatibility between revelation and physicalism is precisely understood depends on how the thesis of revelation and the doctrine of physicalism are respectively cashed out. We have formulated the thesis of revelation, but we have not clarified what physicalism precisely entails. While there is no uncontentious definition of physicalism (see Stoljar, 2010), most physicalists would agree that the phenomenal properties of conscious experiences are purely physical or functional – a physical/functional description exhausts the nature of qualia. So according to physicalism standardly conceived, the only truth that captures the essence of Q is some physical or functional truth, i.e. "Q is X," where "X" is a physical or functional predicate, as in, for instance, "Phenomenal redness is R-fiber firing in the brain," "Itchiness is functional role F (realized by physical properties P1, P2, P3, ...)," etc. In cashing out the incompatibility between revelation and physicalism, I will focus on standard versions of physicalism, namely, versions according to which qualia have physical or functional essences. The structure of this section is as follows. First, I address Lewis' remarks on the matter. Then, I turn to consider role functionalist versions of physicalism. I shall argue that role functionalism is incompatible with revelation. Finally, I lay out a formulation of the incompatibility between the thesis of revelation and standard versions of physicalism. 6.1. Lewis, Revelation and Physicalism Lewis (1995: 142) says that the thesis of revelation is part of the folk psychological concept of qualia and "seems obvious" (that is, intuitive). However, he rejects revelation because it is incompatible with physicalism, which he commits to. After spelling out the thesis of revelation, Lewis moves on to discuss why physicalists cannot accept revelation. He explains: If, for instance, Q is essentially the physical property of being an event of C-firing, and if I identify the qualia of my experience in the appropriate "demanding and literal" sense, I come to know that what is going on in me is an event of C-firing. Contrapositively: if I identify the quale of my experience in the appropriate sense, and yet know nothing of the firing of my neurons, then the quale of my experience cannot have been essentially the property of being an event of C-firing. A materialist cannot accept the Identification Thesis. If qualia are physical properties of experiences, and experiences in turn are physical events, then it is certain that we seldom, if ever, identify the qualia of our experiences. Making discoveries in neurophysiology is not so easy! (op. cit.) The idea here should be straightforward. Recall that for Lewis (1995: 142), the term "qualia" is "a name for the occupants of a certain functional role that is spelled out in our tacitly known folk psychology." We have seen that Lewis allows that the thesis of revelation (his "identification thesis" – component (iv) in section 2) describes one component of the qualia-role. Let Q be the phenomenal property of a pain-experience (its painfulness), and let X be Q's essence and the predicate "X" capture Q's essence. Physicalists like Lewis take X to be a physical property that plays the functional role of quale Q. So "X" is a physical predicate, e.g. "being an event of C-fibers firing." But if the functional role of qualia includes the thesis of revelation – that is, includes our being in a position to know the essences of qualia simply by being the subject of experiences that have the qualia among their properties – then we will not be able to find any physical properties that fulfil the role. In having an experience with quale Q, there is no physical property X such that just by having the experience alone, one can know the truth "Q is X," where "X" is a physical predicate that captures the essence of Q. In having a painexperience, I am certainly not in a position to know de dicto that the painfulness of my experience of pain is its being an event of C-fibers firing. What this means is that no physical property can play the full qualia-role, because no physical property can play the revelation component of the qualia-role. As a result, Lewis drops the thesis of revelation from the full-blown functional definition of qualia which we saw in the beginning of section 2. What remains then are components (i), (ii) and (iii). (i) Qualia cause our "abilities to recognize and to imagine experiences of the same type"; (ii) Qualia are "responsible for responses of pleasure, disgust, etc."; (iii) Qualia are "responsible for judgements of similarity-distance, e.g., the judgement that these two colour-samples nearly match whereas those two contrast strikingly." These remaining components constitute the nearest functional role which some physical properties can indeed satisfy. So, to the question "Should a materialist believe in qualia?" Lewis responds: Yes: he should believe in imperfect but good-enough deservers of the name, occupants of the part of the folk-psychological role we get by leaving out the Identification Thesis [i.e. revelation]. And no: he should not believe in perfect deservers of the name, occupants of the entire role. (Lewis, ibid.: 142-3) As a physicalist, Lewis denies that there is anything that perfectly satisfies our folkpsychological conception of qualia. But he proposes that physical properties of physical events (such as the property of being an event of C-fibers firing) can deserve – imperfectly, but well enough – the name "qualia." 6.2. Revelation and Functionalism As we saw, Lewis is a functionalist who takes qualia to be occupants of functional roles spelt out in folk psychology. As we also saw, Lewis subscribes to what is known as "the identity theory," according to which qualia are just physical properties such as being an event of C-fibers firing. Lewis is thus a realizer functionalist, who identifies qualia with physical properties that realize the corresponding functional roles (see also Lewis, 1966).12 Realizer functionalism is distinguished from role functionalism, which identifies qualia with functional role properties that may be multiply realized by different physical properties playing the same functional role (e.g. in differently constituted creatures). In our discussion on Lewis, we saw that revelation is incompatible with realizer functionalist physicalism and the claim that qualia have physical essences. More needs to be said about whether revelation is incompatible with role functionalist physicalism and the claim that qualia have functional essences.13 In the remainder of this section, by "functionalism," I simply mean role functionalism. Note that different versions of functionalism cash out the notion of a functional role differently. According to scientific functionalism or psychofunctionalism, in the terminology of Block (1978), functional roles are cashed out in the vocabulary of cognitive science or neuroscience. According to what is known as "commonsense functionalism," functional roles are cashed out in the non-technical, everyday vocabulary of commonsense psychology. It is straightforward that revelation is incompatible with psychofunctionalism. In having an experience with a particular quale Q, there is no functional property X cashed out in technical, scientific vocabulary such that just by having the experience alone, one can know that Q is X. It is not so straightforward that the thesis of revelation is incompatible with commonsense functionalism. All else equal, being in a position to know a piece of everyday commonsense seems more likely than being in a positon to know something technical and scientific. A commonsense functionalist would say that "X" is a functional predicate couched in the non-technical vocabulary of commonsense psychology. She might also insist that one is indeed in a position to know such a functional truth "Q is X" by having an experience with quale Q.14 However, I shall argue that revelation is incompatible with commonsense functionalism.15 In cashing out the thesis of revelation, we mentioned the idea that the essence-capturing truth "Q is X" may be hard to put into words. This is supposed to capture the intuitive idea that phenomenal experiences have indescribable phenomenology. 16 Now commonsense functionalists claim that qualia can be fully defined by their functional roles. One might think that intuitively, such functional roles do not seem to capture the "je ne sais quoi (I know not what)" character of qualia.17 But if the essence of a quale is functional (or physical), then it can be put into words. Commonsense functionalists would of course deny that any qualia are indescribable and insist that if one states the full functional role of a quale in the everyday vocabulary of commonsense psychology, then one has successfully given a definition of what that quale is essentially. In spelling out the incompatibility between revelation and commonsense functionalism, I shall grant that the thesis of revelation does not entail, but merely allows, that the essence-capturing truth "Q is X" is hard to put into words.18 I shall also set aside the general concern that, intuitively, a state's having a certain functional role does not guarantee that the state have any phenomenology at all. In order to satisfy the thesis of revelation, the commonsense functionalist would have to say that by having a pain experience with phenomenal property Q (where Q is the painfulness of pain), a subject is in a position to know the commonsense functional role of Q. That is, the subject is in a position to achieve de dicto knowledge of the form "Q is X," where "X" specifies the commonsense functional role of painfulness, which captures the essence of painfulness. I shall argue that in having a pain experience with phenomenal property Q, one is not in a position to know the commonsense functional role of Q; one is not in a position to know de dicto such a truth "Q is X," where the predicate "X" captures the essence of Q in commonsense psychological terms. Commonsense functionalists define qualia in terms of their functional roles by drawing on what is common knowledge about mental states with these qualia (Braddon-Mitchell and Jackson, 2007: 52). According to these functionalists, this common knowledge has three parts: "there are input clauses – clauses that say what sorts of events cause mental states in people; output clauses – clauses that say what sort of behaviours are caused by mental states; and internal role clauses – clauses that describe the internal interactions of mental states" (Braddon-Mitchell and Jackson, ibid.: 48). The input clauses for quale pain would include the clause "bodily damage causes experiences with quale pain"; output clauses would include the clause "experiences with quale pain cause bodily movements that relieve pain and minimize damage"; internal clauses would plausibly include the clause "experiences with quale pain cause desires to relieve pain, which typically lead to desires to take painkillers (if one also believes that taking painkillers would be likely to relieve the pain)" (Braddon-Mitchell and Jackson, ibid., 52-3). Giving a definition of what quale pain is in commonsense functional terms requires specifying all three kinds of clauses. As we saw in section 3, according to the thesis of revelation, the knowledge de dicto that Q is X, where the predicate "X" captures the essence of Q, is gained in virtue of having the experience alone. Let us agree that one is in a position to know the tripartite commonsense functional role of quale pain. Still, it seems implausible that it is in virtue of having a pain experience alone, that one is in a position to have such knowledge, which, according to commonsense functionalists, captures the essence of quale pain. Consider, first, the internal clauses which say that the painfulness of pain causes a desire to relieve pain and motivation toward actions that the subject believes would make the painfulness of pain more likely to stop. The details of such actions and motivations (to take painkillers, for example), are unlikely to be known just in virtue of having a painful experience. If "causing the desire to take painkillers" is built into the internal clauses that specify part of the functional role of quale pain, then the functional role will not be revelation compatible, as we shall say. It is just not true that in virtue of having a pain experience alone, one is in a position to know that quale pain is X where the predicate "X" includes "painkillers." Of course, the commonsense functionalist has a ready response to this point. They should say that what is core and primary in the internal clauses of the functional role of quale pain is nothing specifically about painkillers, but just that the painfulness of pain causes the desire that it stop. Consider, second, the input clauses of the functional role of quale pain which say that bodily damage causes experiences with quale pain. It does not seem to be the case that in virtue of having a pain experience, that is, in virtue of feeling pain, I am in a position to know that it is bodily damage that causes my experience to have a particular phenomenal character. Nothing about bodily damage is revealed to one in having pain experiences like headaches, stomachaches, cramps etc. Even in cases where one feels pain in one's finger as a result of its being cut, having the sensation of pain in one's finger does not on its own reveal that one has a cut on one's finger which gives rise to the sensation. According to proponents of commonsense functionalism, the input clauses that form part of the definition of quale pain explicitly mention "bodily damage" (Braddon-Mitchell and Jackson, ibid.: 52). But a functional role with such input clauses will not be revelation compatible. In having the phenomenal experience of pain alone, one is not typically in a position to know that quale pain is X where the predicate "X" includes "bodily damage." Now, we can grant that there are commonsense clauses about quale pain that one is in a position to know in virtue of having a painful experience, including the output clause "the experience with quale pain causes one to wince, to have reflexive movements of avoidance or withdrawal," the internal clause "the experience with quale pain causes one to want the experience to stop," and perhaps others. But it is far from obvious that the clauses of this reduced, revelation-compatible functional role specification are adequate to capture the essence of quale pain. Specifically, these functional role clauses might not distinguish the painfulness of pain from other qualia. For example, the phenomenal properties of experiences such as biting into a slice of lemon, seeing something extremely bright, hearing a loud fire alarm going off, etc. all typically cause one to wince and to have reflexive movements of avoidance or withdrawal. In all these cases, the subject also typically desires the relevant experience to stop. But while these experiences are examples of distress, they are typically not considered as experiences of pain – one would not usually say that one is in pain if one has one of these experiences. The claim that commonsense role functionalism is compatible with the thesis of revelation clearly faces a dilemma. If a functional role specification is to capture the essence of quale pain, then it must individuate quale pain, in the sense that no other quale has that functional role. A functional role specification that includes enough of what is commonly known about the role of pain to meet this individuation requirement is plausibly not revelation compatible. And a functional role specification that is sufficiently reduced to be revelation compatible is plausibly no longer individuative. This pattern certainly generalizes beyond the example of the painfulness of pain. Consider how a commonsense functionalist would spell out the functional roles of the phenomenal properties of various color experiences. In his paper "Naming the Colours," Lewis (1997) mentions the following two clauses as being central to the commonsense functional roles of worldly colors and corresponding properties of color experiences: D1 Red is the surface property of things which typically causes experience of red in people who have such things before their eyes. D2 Experience of red is the inner state of people which is the typical effect of having red things before the eyes. (Lewis, ibid.: 327) D1 and D2 inter-define red and experience of red. But exactly similar clauses inter-define green and experience of green. So the putative functional role of the pair <red, experience of red> does not individuate that pair. The pair <red, experience of red> is very far from being the only pair <X, Y> such that X is the surface property of things which typically causes Y in people who have such things before their eyes and Y is the inner state of people which is the typical effect of having X things before the eyes. In order to break out of the circle of inter-definition and distinguish experience of red from other colour experiences, D2, which is the input clause for experience of red, needs to be augmented with something about common examples of worldly surfaces that are red, and so (according to D1) cause experience of red. But it seems that having an experience of red does not, by itself, put the subject in a position to know about such examples. For instance, one might define experience of red as being "typically caused by the color of a British pillar box" (Lewis, ibid.: 335). But surely, in having an experience of red alone, one is not in a position to know anything about British pillar boxes nor in a position to know that phenomenal redness is typically caused by seeing a British pillar box.19 This second example helps to sharpen the dilemma facing commonsense functionalists who claim that their position is compatible with the thesis of revelation. If commonsense functionalists define qualia in functional terms by appealing to everyday concepts of worldly objects and states like "painkillers" or "bodily damage" in the case of quale pain and "British pillar box" in the case of phenomenal redness, then commonsense functionalism is incompatible with revelation. But if commonsense functionalists avoid making references to these things in spelling out the commonsense functional roles of qualia, then it seems no longer clear that, with only the vocabulary that would not create a problem for maintaining revelation, they can give adequate functional definitions that individuate qualia. It does not seem that the reduced functional definition of a given quale Q, which the subject is in a position to know by having an experience with quale Q, is fine-grained enough to distinguish Q from other qualia.20 Given this dilemma, our conclusion must be that commonsense role functionalism – like realizer functionalism (and the identity theory) and scientific role functionalism (psychofunctionalism) – is incompatible with the thesis of revelation. 6.3. The Argument from Revelation against Physicalism At this point, it would be useful to formally lay out the incompatibility between the thesis of revelation and physicalism. Here I will present the incompatibility between the two in the format of an argument, namely, an argument from the thesis of revelation to the falsity of physicalism. (Here physicalism is understood as entailing the claim that qualia have physical or functional essences. If there are positions that do not entail this claim, but are nonetheless worthy of the name "physicalism," then whether those versions of physicalism are incompatible with revelation is a separate question.21) (1) If S has an experience-token with quale Q, then S is in a position to know de dicto that "Q is X," where the predicate "X" captures the essence of Q. (2) If physicalism is true, then qualia have physical/functional essences. (3) If qualia have physical/functional essences, then by having an experiencetoken with quale Q, S should be in a position to know de dicto that "Q is X," where "X" is a physical/functional predicate which captures the essence of Q. (4) It is not true that by having an experience-token with quale Q, S is in a position to know de dicto that "Q is X," where "X" is a physical/functional predicate which captures the essence of Q. (5) Physicalism is false. Premise (1) is entailed by the thesis of revelation. (2) states what the essences of qualia would be if physicalism were true. (3) follows from (1) and (2). (4) is supported by the considerations discussed in sections 6.1 and 6.2. (5) then concludes from (3) and (4) that physicalism is false. Given our clarification of the distinction between knowledge de dicto and knowledge de re of qualia in section 4, it is worth noting that physicalists, in denying revelation, can still say that by having an experience with quale Q, one is in a position to know de re of X, where the predicate "X" captures the essence of Q, that Q is it (see Lewis, 1995: 143). A physicalist might say that there are two distinct modes of presentation of the essence of the phenomenal property Q – a physical/functional mode of presentation and a phenomenal mode of presentation, and that the mode of presentation that reveals the essence of Q is the physical/functional one (see Loar, 1997: 603). So "X," which captures the essence of Q, is a physical/functional predicate. Such a physicalist can say that by having an experience with quale Q, one is in a position to know de re of physical/functional property X (but under its phenomenal mode of presentation) that Q is it, e.g. know de re of the property of being an event of C-fibers firing that the painfulness of a pain experience is that property.22 This is analogous to the case of Tom's knowledge of Venus. Suppose again that Tom thinks of Venus as Phosphorus and not as Venus and that Tom knows de dicto that Hesperus is Phosphorus. In such a case, we can still say that "Tom knows of Venus that Hesperus is it." Tom has knowledge de re of Venus (even without any idea what Venus is) insofar as he thinks of Venus as Phosphorus. One can also see that the doctrine of the simplicity of qualia, which we discussed in section 5, is in tension with physicalism. If qualia are physical or functional properties, which are presumably not simple, then qualia cannot be simple. Hence, physicalists also ought to reject the idea that qualia are simple (see Lewis, 1995: 142). In section 5, we saw that there is an argument from revelation to the conclusion that some qualia are simple. The argument crucially relies on the premise that there is at least a range of cases such that in having an experience with Q, one is not in a position to know that Q has internal structure. A commonsense functionalist who claims their position to be compatible with revelation is likely to reject this premise and claim that in all cases, by having an experience with Q, one is indeed in a position to know the complex structure of Q as a result of being in a position to know the complex commonsense functional role of Q.23 This allows such a commonsense functionalist to reject the entailment from the thesis of revelation to the claim that some qualia are simple. But as we saw, it is unclear that commonsense functionalism is compatible with revelation in the first place. 7. Conclusion In this paper, we have offered a detailed discussion on the thesis of revelation and its incompatibility with physicalism. Much of our discussion focused on Lewis' remarks on revelation. Despite Lewis' illuminating discussion in his 1995 paper, it is only more recently that the thesis of revelation has been brought up explicitly in relation to the discussion on whether physicalism is true (see Stoljar, 2006, 2009; Majeed, 2017; Trogdon, 2017; Goff, 2017).24 But in the literature, as already mentioned, revelation and physicalism are not always thought to be incompatible (see Damnjanovic, 2012; Stoljar, 2018). Of course, the issue here depends on how one cashes out the thesis of revelation and how one formulates physicalism. We have just seen that the thesis of revelation, as we have formulated it here, is indeed incompatible with standard versions of physicalism, which take qualia to have physical/functional essences. Whether the argument from revelation against physicalism is compelling depends, of course, on whether the thesis of revelation itself is compelling, an issue that we have not touched on in this paper. At the beginning of the paper, I mentioned that revelation is often thought of as intuitive, even by avowed physicalists. But the claim that revelation is intuitive is not without contention (see Stoljar, 2009). Whether revelation is intuitive, and in what sense it is intuitive if it is, are issues that await further discussion. I am inclined to think that revelation is intuitive in the sense that it is part of our ordinary conception of experience, and that this is evident from the way we talk about experiences.25 Even if revelation turns out to be an intuitive claim, whether it is in fact true is a separate question. There are few direct arguments against revelation in the literature.26 Physicalists like Lewis, who acknowledge the intuitiveness of revelation, end up denying it simply because of its incompatibility with physicalism. Further discussion on the plausibility of revelation is thus called for.27 References Adams, Robert Merrihew. "Flavors, Colors, and God." 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"The Content of Phenomenal Concepts." In The Character of Consciousness, 251-276. Oxford: Oxford University Press, 2010. Cottingham, John. "Intentionality or Phenomenology? Descartes and the Objects of Thought." In History of the Mind-Body Problem, edited by Tim Crane and Sarah Patterson, 139-55. London: Routledge, 2012. Damnjanovic, Nic. "Revelation and Physicalism." Dialéctica 66, no. 1 (2012): 69-91. Descartes, René. The Philosophical Writings of Descartes, Volume 1. Translated by John Cottingham, Robert Stoothoff and Dugald Murdoch. Cambridge: Cambridge University Press, 1985. Díaz-León, Esa. "Do A Posteriori Physicalists Get Our Phenomenal Concepts Wrong?." Ratio 27, no. 1 (2014): 1-16. Elpidorou, Andreas. "A Posteriori Physicalism and Introspection." Pacific Philosophical Quarterly 97, no. 4 (2016): 474-500. Fine, Kit. "Essence and Modality: The Second Philosophical Perspectives Lecture." Philosophical Perspectives 8 (1994): 1-16. Gewirth, Alan. "Clearness and Distinctness in Descartes." Philosophy 18, no. 69 (1943): 17-36. Goff, Philip. "A Posteriori Physicalists Get Our Phenomenal Concepts Wrong." Australasian Journal of Philosophy 89, no. 2 (2011): 191-209. ---. "Real Acquaintance and Physicalism." In Phenomenal Qualities: Sense, Perception, and Consciousness, edited by Paul Coates and Sam Coleman, 121-43. Oxford Oxford University Press, 2015. ---. Consciousness and Fundamental Reality. Oxford University Press, 2017. Hill, Christopher S. Meaning, Mind, and Knowledge. Oxford: Oxford University Press, 2014. Hume, David. A Treatise of Human Nature, edited by David Fate Norton and Mary J. Norton. Oxford: Oxford: Oxford University Press, 2000. Johnston, Mark. "How to Speak of the Colors." Philosophical studies 68, no. 3 (1992): 221-63. Levine, Joseph. Purple Haze: The Puzzle of Consciousness. Oxford University Press, 2001. Lewis, David. "An Argument for the Identity Theory." The Journal of Philosophy 63, no. 1 (1966): 17-25. ---. "Should a Materialist Believe in Qualia?." Australasian Journal of Philosophy 73, no. 1 (1995): 140-44. ---. "Naming the Colours." Australian Journal of Philosophy 75, no. 3 (1997): 32542. Loar, Brian. "Phenomenal States." In The Nature of Consciousness, edited by Owen J. Flanagan and Güven Güzeldere Ned Block, 597-616. Massachusetts: MIT Press, 1997. Majeed, Raamy. "Ramseyan Humility: The Response from Revelation and Panpsychism." Canadian Journal of Philosophy 47, no. 1 (2017): 75-96. McLaughlin, Brian P. "In Defense of New Wave Materialism: A Response to Horgan and Tienson." In Physicalism and Its Discontents, edited by Carl Gillett and Barry Loewer, 319-30. Cambridge: Cambridge University Press, 2001. ---. "Color, Consciousness, and Color Consciousness." In Consciousness: New Philosophical Perspectives, edited by Quentin Smith and Aleksandar Jokic, 97-154. Oxford: Oxford University Press, 2003. Nida-Rümelin, Martine. "Grasping Phenomenal Properties." In Phenomenal Concepts and Phenomenal Knowledge: New Essays on Consciousness and Physicalism, edited by Torin Alter and Sven Walter, 307-36. Oxford: Oxford University Press, 2007. Papineau, David. "Phenomenal and Perceptual Concepts." In Phenomenal Concepts and Phenomenal Knowledge: New Essays on Consciousness and Physicalism, edited by Torin Alter and Sven Walters, 111-44. Oxford: Oxford University Press, 2007. Patterson, Sarah. "Clear and Distinct Perception." In A Companion to Descartes, edited by Janet Broughton and John Carriero, 216-34, 2008. Schroer, Robert. "Where's the Beef? Phenomenal Concepts as Both Demonstrative and Substantial." Australasian Journal of Philosophy 88, no. 3 (2010): 505-22. Sosa, Ernest. "Propositional Attitudes De Dicto and De Re." The Journal of Philosophy 67, no. 21 (1970): 883-96. Stanley, Jason, and Timothy Willlamson. "Knowing How." The Journal of Philosophy 98, no. 8 (2001): 411-44. Stoljar, Daniel. Ignorance and Imagination: The Epistemic Origin of the Problem of Consciousness. Oxford: Oxford University Press, 2006. ---. "The Argument from Revelation." In Conceptual Analysis and Philosophical Naturalism, edited by David Braddon-Mitchell and Robert Nola, 113-38. Cambridge: MIT Press 2009. ---. Physicalism. London: Routledge, 2010. ---. Review of Consciousness and Fundamental Reality, by Philip Goff. Notre Dame Philosophical Reviews, February 9, 2018. Strawson, Galen. "Red and 'Red'." Synthese 78, no. 2 (1989): 193-232. Taylor, Henry. "Powerful Qualities, Phenomenal Concepts, and the New Challenge to Physicalism." Australasian Journal of Philosophy 96, no. 1 (2018): 53-66. Trogdon, Kelly. "Revelation and Physicalism." Synthese 194, no. 7 (2017): 2345-66. 1 Both can be read as endorsing versions of revelation. Descartes (1985: 215, 217) thought that upon a careful reflection, we are capable of having clear and distinct ideas of experiences such as sensations. To have a clear and distinct idea of something, according to the standard interpretation of Descartes, is to know, at least to some extent, what belongs and does not belong to the essence of that thing (see Gewirth, 1943; Patterson, 2008). One might also read Hume as advocating a thesis of revelation. Hume (2000: 9, 136) thought that impressions, which are roughly feelings, including "sensations, passions and emotions," "appear, all of them, in their true colours" and "must necessarily appear in every particular what they are, and be what they appear." 2 Of course, any experience-token belongs to many experience-types, of varying degrees of specificity. When we talk about experience-tokens of the same experience-type, we rely on context to indicate the relevant degree of specificity. 3 Revelation is about knowing the essence of phenomenal properties. Although in his original paper Fine was mainly concerned with individual-essence, one can extend the Finean conception of essence to apply to type-essence. 4 Here knowledge-what is understood as a kind of knowledge-that. This is the orthodox approach to knowledge-what in the literature (see Stanley and Williamson, 2001). Knowing what x is at least involves knowing a proposition which is the answer to the question "What is x?" 5 Immediately after the passage quoted above, Lewis writes: If, for instance, Q is essentially the physical property of being an event of C-firing, and if I identify the qualia of my experience in the appropriate 'demanding and literal' sense, I come to know that what is going on in me is an event of C-firing. Contrapositively: if I identify the quale of my experience in the appropriate sense, and yet know nothing of the firing of my neurons, then the quale of my experience cannot have been essentially the property of being an event of C-firing. (Lewis, 1995: 142) Note that being an event of C-firing is, strictly speaking, not a property of the quale Q which is the qualitative character of pain-experience, i.e. quale pain. The property being an event of C-firing is more appropriately understood as a property of pain-experience-token itself. 6 On some accounts, our rich understanding of phenomenal properties is embodied in a demonstrative-like, or recognition-based, phenomenal concept (e.g. Chalmers, 2010). The thesis of revelation, in recent literature, has been formulated via the notion of phenomenal concepts (see Goff, 2015, 2017; Trogdon, 2017). The present formulation does not depend on the notion of phenomenal concepts and captures the intuitive idea behind the thesis. 7 The same distinctions apply to other propositional attitudes such as belief, desire, etc. 8 Knowledge de dicto and knowledge de re are not mutually exclusive. For instance, I know that the sun will rise tomorrow. I not only have knowledge with respect to a dictum, namely, "The sun will rise tomorrow," I also have knowledge with respect to a res, namely, the sun. 9 In having knowledge de dicto that "Hesperus appears in the evening sky," Tom also has knowledge de re of Hesperus/Venus. Such knowledge de re can also be reported with a knowledge-attribution de dicto – in this case, namely, "Tom knows that Hesperus appears in the evening sky." 10 According to Brian Loar (1997), the word "capture" is ambiguous. Loar (ibid.: 603) points out two different senses in saying that "a concept captures the essence of the referent." He writes: These are equivocating uses of "capture the essence of." On one use, it expresses a referential notion that comes to no more than "directly rigidly designate." On the other, it means something like "be conceptually interderivable with some theoretical predicate that reveals the internal structure of" the designated property. But the first does not imply the second. (op. cit.) For Loar, who is an a posteriori physicalist, a phenomenal concept can be said to capture the essence of the phenomenal property in the sense that it directly refers to the referent, but it does not tell us anything about the nature of the referent. This is not how I have intended to use the term "capture." On the other hand, a physicalist like Loar would say that a physical concept captures the essence of the phenomenal property, in the sense that it tells us what the property really is in a way that reveals the nature of the property. This is the sense of "capture" at issue here. The thesis of revelation says that the truth "Q is X," which one knows through having an experience of the type Q and which is hard to put into words, is a truth that captures, in this second sense, the essence of Q. 11 It seems that whenever we notice our phenomenology to have structures, we can describe the phenomenology, including its internal structure, to some extent which goes beyond using a mere demonstrative "this." We might say things like: "There is a bit of phenomenal redness on the left of my visual field. Next to it, there is some phenomenal blueness, etc." From this, it does not follow that a complex, structured experience can be fully captured in a verbal description. The simple constituent phenomenal properties will likely not be captured in words without resorting to demonstratives and/or metaphors. 12 Lewis is also an a priori physicalist, who holds the view that the conditional "If P then Q," where P stands for the totality of physical truths and Q stands for a given phenomenal truth, is true a priori. One can also be an a posteriori physicalist and hold the view that "If P then Q" is only true a posteriori. Many proponents of a posteriori physicalism explicitly reject revelation (see Loar, 1997; McLaughlin, 2001, 2003; Papineau, 2007; Balog, 2012). Like Lewis, they reject it simply because it renders physicalism false. 13 Both identity theorists and realiser functionalists would agree that phenomenal properties have physical essences. In contrast, role functionalists take phenomenal properties to have functional essences. 14 I am indebted to Frank Jackson for pressing me on this issue. 15 There are problems with commonsense functionalism that are independent of the thesis of revelation (see Block, 1978). 16 The idea that qualia are indescribable can be traced to Descartes, who thinks that sensations have an "Iknow-not-what" ("je ne sais quelle" in French) aspect (see Cottingham, 2012). For a recent discussion on the indescribable character of experience, see Camp (2006). 17 This, I think, points to the common objection against functionalism, that knowing what the functional role of pain is does not seem to amount to knowing what pain feels like, and the essence of the painfulness of pain does not seem to be exhausted by its functional role. 18 The idea that mere words are unable to communicate the essence of phenomenal property Q is not officially part of the thesis of revelation. In cashing out revelation, I merely left open the possibility that the essence-capturing truth "Q is X" may be hard to put into words. As we have seen, the reason that the identity theory or realizer-functionalist physicalism is incompatible with the thesis of revelation is not that revelation requires the essences of qualia to be indescribable – it does not explicitly require that. 19 It is interesting to note that in his paper "Naming the Colours," Lewis (1997: 338-9) discusses the idea that adding revelation to the functional definition of colour experience would help individuate experience of red and distinguish it from other colour experiences. However, he rejects this approach because he takes revelation to be false by materialistic lights. 20 Block (1978: 300) raises a similar worry that common knowledge can fail to distinguish two sensations that feel different. 21 For instance, one might argue that physicalism defined as a grounding thesis does not entail that qualia have physical/functional essences (see Stoljar, 2018). Whether revelation is incompatible with grounding physicalism, i.e. physicalism cashed out as a grounding thesis, requires separate treatment. 22 A physicalist can say that the essence of Q, which is a physical/functional property, can be thought about in a physical/functional way, i.e. as X, or in a phenomenal way, as, say, Y. Such a physicalist can say that by having an experience with quale Q, one is in a position to know de dicto that Q is Y. Note that "Y," which is a phenomenal predicate, refers to but does not capture the essence of Q. 23 Note that such a commonsense functionalist operates on a broad conception of qualia where the functional roles of qualia form at least part of their essences. One might say that on a narrow conception of qualia which treats functional roles as being separate from the essences of qualia, we do not notice some qualia to have structures in having the relevant experiences. 24 One does find something similar to the thesis of revelation being used as an objection against the phenomenal concept strategy (see Levine, 2001: 84; Nida-Rümelin, 2007; Goff, 2011, 2015). This is the worry that the phenomenal concept strategy cannot account for the idea that phenomenal concepts afford us substantive knowledge about the nature of phenomenal properties. For replies to this challenge from a posteriori physicalists, see Schroer (2010), Díaz-León (2014), Elpidorou (2016), and Taylor (2018). 25 In an unpublished paper, I put forward an argument for the intuitiveness of revelation which takes the form of an inference to the best explanation. I contend that sentences such as "I know what pain feels like, but I don't know what the feeling of pain really is" are odd, and such oddity is best explained by the hypothesis that the thesis of revelation is part of our ordinary conception of experience. Given this inference to the best explanation, revelation is part of our ordinary conception of experience and is thus intuitive. 26 Recently, Stoljar (2018) in his review of Goff's book Consciousness and Fundamental Reality, has raised an argument against the thesis of revelation. I am not aware of any other such arguments. 27 I am very grateful to Gonzalo Rodriguez-Pereyra, Bill Child and especially Martin Davies for their helpful comments on drafts of this paper.