"Leibniz's Wolffian Psychology" Corey W. Dyck In spite of the fact that his signature metaphysical doctrine is built on the model of the soul, Leibniz never published a text devoted to psychology nor does he even offer much in the way of a sustained discussion of the human soul in his published or unpublished texts. This fact has not stopped scholars from identifying Leibniz as one of the founders (along with Descartes) of the discipline of rational psychology, understood as an investigation of the soul that proceeds purely a priori. Leibniz undoubtedly has his moments as a rational psychologist in this narrow sense-the famous "mill argument" of the Monadology serving as a ready example of deploying the resources of reason to derive a claim about the soul, and which argument appears to be targeted by Kant in his devastating criticism of the rational investigation of the soul in the Paralogisms.1 Even so, Leibniz never to my knowledge codifies this as the methodology of rational psychology, nor does he ever disqualify observation as a source of cognition regarding the soul or indeed even discourage the analogy, founded on his doctrine of the pre-established harmony, between the investigation of physical and of psychical phenomena. One would thus be justified in wondering whether Leibniz's identification as a narrowly rationalist psychologist can be borne out by a wider survey of his texts. In this paper, I will attempt to trace the broader contours of a putative Leibnizian psychology. To achieve this, however, I will adopt the rather unusual, and perhaps historically dubious, strategy of outlining the continuities between Leibniz's discussion of the soul and the much more detailed and systematic psychological writings of his German successor, Christian Wolff. While Wolff's writings on psychology appeared only after Leibniz's death, and indeed could draw on little of Leibniz's writings, I will argue here that a consideration of Leibniz's discussion of the soul in light of Wolff's later treatment serves to bring important details and unnoticed continuities into focus and ultimately reveals a different profile for Leibniz's psychology. I will begin, then, with a consideration of Wolff's rational and empirical psychology. I will then turn to a consideration of some representative treatments of psychological themes in Leibniz's corpus with the aim of illustrating the continuity between his and Wolff's treatments. I will conclude with some brief remarks on the possible source and broader significance of this evident continuity. 1 See Margaret Dauler Wilson: "Leibniz and Materialism", in: Canadian Journal of Philosophy, 3(4) (1974), 495–513. 2 Christian Wolff's rational psychology is frequently mischaracterized as a narrowly rationalistic enterprise, that is, as proceeding completely independently of experience. Yet a look at Wolff's numerous discussions of the doctrine of the soul throughout his writings reveals not only that Wolff draws liberally upon the observations and experiences of the soul catalogued in empirical psychology in the context of its rational counterpart, but also that Wolff conceives of rational psychology as a mixed discipline and indeed as one proceeding in accordance with the model of natural science. We might begin with Wolff's account of the basis of our cognition of our existence as presented in the initial sections of his Deutsche Metaphysik in the form of the following syllogism: Whoever is conscious of himself and of other things, is. We are conscious of ourselves and other things. Therefore, we exist. (DM §6)2 The major of this syllogism is, according to Wolff a self-evident principle: insofar as we recognize that we are conscious, our existence also follows since it cannot be doubted that "a thing exists of which we cognize that it is in some specific way" (DM §7). The minor premise, by contrast, is an empirical claim which Wolff characterizes as an "indubitable experience [unzweifelbare Erfarhung]." This claim, oddly formulated in the first-person plural, is nonetheless justified given that the experience of the fact that one is conscious is available to anyone and is not such that it could differ between subjects (cf. DL c. 5, §2). It is on account of the empirical character of the minor premise (as well as the conclusion) that Wolff later includes this demonstration at the outset of his Psychologia empirica (§§11–17). Even so, this does not prevent Wolff from emphasizing the significance of this cognition for rational psychology, where it serves to demonstrate the existence of the soul or the subject of consciousness. Having defined the soul as "that thing which is conscious of itself and other things outside of it" (PE §20; cf. also DM §192), which is taken to imply, in accordance with 2 For ease of reference, I have made use of the following abbreviations for frequently cited texts: (works by Descartes) [AT] Oeuvres de Descartes, edited by Charles Adam and Paul Tannery (Paris 1897–1909); [CSM] The Philosophical Writings of Descartes and Correspondence, edited and translated by John Cottingham, Robert Stoothoff, Dugald Murdoch, and Anthony Kenny (Cambridge 1984–1991); (works by Leibniz) [AG] Philosophical Essays, ed. and trans. Roger Ariew and Daniel Garber (Indiana 1989); [BLW] Briefwechsel zwischen Leibniz und Christian Wolff: Aus den Handschriften der koeniglichen Bibliothek zu Hannover, ed. C. I. Gerhardt (Halle 1860); [L] Philosophical Papers and Letters, a selection translated, edited with an introduction by Leroy E. Loemker, 2nd edn. (Dordrecht 1989); [NE] New Essays on Human Understanding, ed. and trans. Peter Remnant and Jonathan Bennett (Cambridge 1981); (works by Wolff) [AN] Ausführliche Nachricht von seinen eigenen Schrifften, die er in deutscher Sprache heraus gegeben, 2nd edn. (Frankfurt am Main: 1733; reprint Hildesheim 1973); [DL] Vernünftige Gedanken von den Kräften des menschlichen Verstandes und Ihrem richtigen Gebrauche in Erkänntnis der Wahrheit or Deutsche Logik (Halle: 1713; reprint Hildesheim 1965); [DM] Vernünfftige Gedancken von Gott, der Welt und der Seele des Menschen, auch allen Dingen überhaupt or Deutsche Metaphysik (Halle: 1751; reprint Hildesheim 1983); [LL] [DP] Philosophia rationalis, sive Logica, 3 vols. (Vol. 1 contains the Discursus praeliminaris de philosophia in genere), 3rd edn. (Frankfurt am Main: 1740; reprint Hildesheim 1983); [PE] Psychologia empirica, 2nd edn. (Frankfurt and Leipzig: 1738; reprint Hildesheim 1968); [PR] Psychologia rationalis, 2nd edn. (Frankfurt am Main: 1740; reprint Hildesheim 1972). All translations from Wolff's texts are my own. 3 the foregoing, that that which is conscious in us is just the soul which must exist given that anything which is thus conscious exists (PE §21), Wolff contends that the demonstration that we exist also amounts to a demonstration of the existence of the human soul which, as the central concept of rational psychology, is taken to amounts to a vindication of the possibility of that discipline (PR §5). That Wolff should thus draw upon an empirical cognition (in this case of the existence) of the soul within his rational psychology is not an aberration, as he consistently stresses the interconnection of empirical and rational psychology. In his original (German) presentation of the two disciplines, Wolff distinguishes between empirical psychology as that discipline which seeks to explain what we perceive of the soul through everyday experiences (DM §191), and rational psychology understood as the doctrine that "is permitted to treat of different things concerning the soul, things to which experience does not so easily lead" (DM §727). Having introduced the distinction between these two approaches to the soul, however, Wolff proceeds to underline the ways in which rational psychology nonetheless relies upon empirical psychology. So, Wolff contends that one of the tasks of the rational investigation is to clarify and organize the cognitions generated in the course of the empirical investigation; thus, in rational psychology, "we will seek distinct concepts among that which we perceive of the soul and note again here and there some important truths that can be proved from these" (DM §191). Moreover, Wolff is clear that empirical psychology provides the principles from which the demonstrations formulated in the rational discipline set out (DM §191). Lastly, rational psychology also relies upon the empirical discipline inasmuch as the rational psychologist must turn to experience to provide the "touchstone" for verifying its conclusions (DM §727). Wolff likewise makes this dependence of the rational discipline upon experience clear in his later Latin textbooks, claiming that rational psychology borrows principles from empirical psychology (among other disciplines; PR §3), and that it is only on the basis of what we are aware of in the soul that rational psychology is warranted in extending our cognition of the soul beyond the boundaries of experience (PR §9). As Wolff summarizes, "Rational psychology obviously expands the space of empirical psychology, while borrowing principles from it: it returns with interest what it has borrowed" (PE §5). Accordingly, for Wolff, rational psychology can hardly be identified as 'rational' in the narrow sense that it proceeds independently of experience. As Wolff explains, some disciplines like arithmetic and geometry are rational sciences just because they employ only pure ("lauter") reason and proceed without any empirical admixture. So, he notes that "reason is pure if in the course of inference nothing is admitted except definitions and propositions cognized a priori" (PE §495). 4 Significantly, however, Wolff distinguishes sciences of pure reason from other sciences which are rational only in a mixed sense: We find enough examples in the sciences that our reason is not always pure, particularly in the cognition of nature and of our selves. Moreover, I take it to be the surest way that we accept in the cognition of nature nothing but what is grounded in infallible experiences. Since those who want to include more into reason than it has a right to come upon things which are the products of fancy and thereby stray from truth into error. (DM §382) As opposed to arithmetic and geometry, which "abstract from the senses" (DM §382) and are therefore products of pure reason, rational psychology is 'rational' only in a mixed sense inasmuch as it incorporates experiences into its demonstrations. Moreover, this experience does not compromise the certainty of the conclusions yielded by rational psychology since the indubitable experiences that can serve as principles in the rational discipline (such as that we are conscious of ourselves and other things) are such that they cannot possibly mislead us. Consequently, that rational psychology is identified as an a priori discipline, does not imply that it abjures all experience in its search for universal and necessary cognitions of the soul (since experience is taken to supply such cognition as well); rather, it simply means that it will proceed by means of inference from what is given empirically of the soul,3 with the result being a distinctively "mixed cognition" which comprises elements that are acquired both a posteriori and a priori (PE §434). Wolff's emphasis on the role of experience in rational psychology reflects his ambition of founding a properly scientific approach to the soul in which both disciplines play a key role. This is signalled, for instance, in Wolff's comparison of empirical and rational parts of psychology to observational and theoretical astronomy inasmuch as, for instance, one discipline formulates predications that are subject to confirmation by our observation (PE §5). Indeed, Wolff takes the properly scientific character of his rational psychology rather seriously, extending even to the deployment of hypotheses in his treatment of the soul. The most important use of this method occurs in the course of Wolff's discussion of the various systems of accounting for the ground of the (observed) agreement between states of the body and states of the soul, namely, the system of natural or physical influence, the "Cartesian" system (of occasional causes), and the system of preestablished harmony. While Wolff endorses the system of pre-established harmony as consistent both with the laws of nature and with the demonstration that the soul has a force (Kraft, vis) of its own (DM §765), he later clarifies that each of these three systems constitutes a probable hypothesis, 3 On Wolff's distinct sense of a priori, see Hans-Jürgen Engfer: "Von der leibnizischen Monadologie zur empirischen Psychologie Wolffs", in: Sonia Carboncini and Luigi Cataldi Madonna (eds.): Nuovi studi sul pensiero di Christian Wolff (Hildesheim 1992), 193–215; here 212. 5 and that none finds definitive confirmation in experience.4 More than simply making use of scientific method in his rational psychology, Wolff is quite explicit in modelling the investigation of the soul after the investigation of external objects. So, the relation between empirical and rational psychology mirrors that between experimental and dogmatic physics: as Wolff notes, rational psychology depends on empirical psychology for its principles just as "dogmatic" physics rely upon experimental physics, or "the part of philosophy that establishes the principles of physics through experiments" (DP §107). Moreover, Wolff upholds a close parallelism between the general methodologies of psychology and physics, contending that both seek to explain phenomena in terms of the activity of an underlying force on the part of some substance: when the reason for corporeal phenomena is to be given from their structure, the reason must also include the rules of motion; similarly, the laws of sensation, imagination, intellection and appetition must be referred to, when the reason for what pertains to the soul is to be given from the force of representing the universe (PR §529) Wolff takes this claim of a parallelism between the methods of psychology and physics as rather innovative, contending that: "it has been held as almost impossible to philosophize about the soul in such a way as we are used to philosophize about corporeal things" (DM Preface to 4th edition, §3). Indeed, the innovative character of Wolff's psychology, both empirical and rational, cannot be denied. As has been explored recently by Vidal, Wolff's borrowings from physics and astronomy for use within the context of rational psychology represents a significant break from the pre-modern Scholastic tradition that still found staunch defenders in late 17th Century Germany.5 Yet, the contrast that would have likely been foremost on Wolff's mind is with the Cartesians, who in upholding a strict dualism between extended and thinking substance effectively ruled out any application of the concepts and principles that hold in the investigation of bodies (whether mundane or heavenly) to the soul. Descartes can be taken to make just such a distinction between the ways in which we investigate the soul and that in which we investigate worldly phenomena in his Reply to Gassendi: You want us, you say, to conduct 'a kind of chemical investigation' of the mind, as we would [for instance] of wine. This is indeed worthy of you, O Flesh, and of all those who have only a very confused conception of everything, and so do not know the proper questions to ask about each thing. (AT VII, 359-60; CSM II, 248-9) 4 See AN §99. See also Hans Werner Arndt: "Rationalismus und Empirismus in der Erkenntnislehre Christian Wolffs", in: Werner Schneider (ed.), Christian Wolff, 1679–1754: Interpretationen zur seiner Philosophie und deren Wirkung (Hamburg 1983), 31–47, 35 and n33; and Richard J. Blackwell: "Christian Wolff's Doctrine of the Soul", in: Journal of the History of Ideas, 22(3) (1961), 339–354, here 348–54. 5 See Fernando Vidal: The Sciences of the Soul: The Early Modern Origins of Psychology (Chicago 2011); chapter 3. 6 For Wolff, by contrast, the world is comprised of a single sort of substance, and the parallelism between series of changes in the soul and those of the body suggests to Wolff that the same general scientific laws (such as laws of motion-cf. DM §737) and concepts (like that of force) hold for phenomena in each domain. Without denying the innovative character of Wolff's empirical and rational psychology, I do want to suggest that viewing Leibniz's scattered discussions of psychology through the lens of Wolff's more systematic presentation reveals unmistakable surprising anticipations of his successor's treatment. As noted, psychological concerns are central to Leibniz's metaphysics given that monads are understood on the model of souls. Even so, Leibniz spends surprisingly little time discussing psychology-'pneumatics' or 'pneumatology' (Leibniz's preferred terms for the doctrine of the soul) are only mentioned a handful of times in his writings, and furthermore it is not clear in these scant references that his understanding of it is even compatible with Wolff's. 'Pneumatology' crops up in the New Essays, where Leibniz claims that it has to do with "knowledge of God, souls, and simple substances in general" (NE Preface, 57). Such a characterization of the discipline, however, assigns it a far broader scope than Wolff's psychology since, as is well known, Wolff departs from Leibniz in remaining agnostic regarding whether all simple substances are endowed with a representative power (and so Wolff excludes them from the purview of psychology); moreover, he limits the scope of his psychology to finite spirits, with God as infinite spirit treated exclusively in the separate discipline of natural theology. Even so, in an unpublished text on the division of philosophy, Leibniz offers an account of pneumatology that comes closer to Wolff's psychology. There, Leibniz identifies psychology as a twofold discipline, with one (unnamed) part concerned with "perceiving things in general," which is to say monads as such, and another concerned with "intelligent things, or spirits," where this latter discipline is identified as pneumatology when it concerns minds, particularly our own (C 526). Taken in this latter sense, Leibniz's pneumatology like Wolff's psychology refers more narrowly to the treatment of finite minds, and even if this definition is not uniformly endorsed throughout Leibniz's corpus it will be useful for the sake of effecting an instructive comparison with Wolff to focus our consideration of Leibniz's discussion accordingly (and so when I speak of Leibniz's 'psychology' it is this characterization of pneumatology that I have in mind). We might begin, as we did with Wolff, with Leibniz's account of the basis for the cognition of our own existence and the role it might play in Leibniz's psychology. As presented above, Wolff offers a reconstruction of the Cartesian cogito in terms of a syllogism which yields an empirical conclusion, and which serves to vindicate the assumption of the reality of the concept of soul at the 7 basis of psychology. Turning to Leibniz's treatment of the cogito we find that he, by contrast, takes the cogito to express an immediate truth rather than something known by means of inference: One can always say that the proposition I exist is evident in the highest degree, since it cannot be proved by anyone else-indeed, that it is an 'immediate truth.' To say I think therefore I am is not really to prove existence from thought, since to think and to be thinking are one and the same, and to say I am thinking is already to say I am. (NE 411) While this represents an undeniable difference from Wolff's (rather unique) treatment of the cognition of our own existence, it should not obscure the deeper continuities between the two when it comes to the characterization of this cognition and its subsequent use. Regarding the former, Leibniz is clear that while he takes the proposition I am thinking for an immediate truth, it is nonetheless "a proposition of fact [...] not a necessary proposition" (NE 411), which is to say that its truth is not founded on the principle of contradiction (cf. GP IV, 357; L 385). Part of the reason for this identification is, as Leibniz explains, that the cogito expresses a feeling or inner experience of my own existence; thus, Leibniz writes that the cogito is "founded on immediate experience" (NE 411) and claims that this cognition is given with the "immediacy of feeling" (NE 367). This brings Leibniz's understanding of the cogito in line with Wolff's, at least as far as its empirical character is concerned, though at one point Leibniz suggests that its identification of a truth of fact could be taken to disqualify it from use as a first principle (NE 411), and so disqualify it from the sorts of uses that Wolff subsequently puts it to in his rational psychology. However, Leibniz explains that the proposition I exist thinking is a 'primary' truth of fact, thereby drawing attention to its peculiar character as a truth which, while not a necessary proposition (or "identity"), is nonetheless certain (NE 434; cf. also 387). Moreover, in virtue of their special status, primary truths of fact can serve as axioms, or propositions that are "immediate and indemonstrable" (NE 408); thus, as Leibniz writes, "if you take axioms, in a more general manner, to be immediate or non-provable truths, then the proposition I am can be called an axiom" (NE 411; cf. also 434). Significantly, this is entirely consistent with, even an anticipation of, Wolff's own explicit allowance for (propositions derived from) experiences serving as principles in demonstrations (DL c. IV, §21; cf. also LL §498).6 Consistent with this grounding of psychology in a (primary) truth of fact, Leibniz frequently characterizes his psychology as a mixed discipline, one that will make use of empirical cognition in order to support even the most abstract claims about the soul. It bears noting first that, in stark contrast with his strictly rationalist reputation, Leibniz emphasizes the key role played by empirical cognition even in the context of metaphysics, a point that is clearest, for instance, in his discussion 6 For more on this, see Corey W. Dyck: Kant and Rational Psychology (Oxford 2014), 22-5. 8 in the New Essays where Leibniz laments that "in metaphysics and ethics there is no longer a parallel between reasoning and experience" (NE 371). Theology in particular serves as an example of the possibility of incorporating experience within a properly metaphysical discipline as it comprises both an empirical and a rational part; as he writes, "Christian theology [...] is founded on revelation, which corresponds to experience" and on "natural theology, which is derived from the axioms of eternal Reason" (NE 415–16). Echoing (or rather, anticipating) Wolff's identification of such a science as a kind of mixed discipline, Leibniz envisages a science consisting of "mixed propositions," though in this case the example is provided by astronomy: there are 'mixed propositions' which derive from premises some of which come from facts and observations while others are necessary propositions. These include a great many of the findings of geography and astronomy about the sphere of the earth and the paths of the stars, arrived at by combining the observations of travellers and astronomers with the theorems of geometry and arithmetic. (NE 446) Generally speaking, then, experience has an important role to play even in metaphysics, indeed, is even indispensable for it, as Leibniz suggests when he claims that for the metaphysician it is experience which "aids and sustains their steps like the little wheeled device which keeps toddlers from falling down" (NE 371). This general emphasis on the importance of experience for metaphysics is likewise clear in Leibniz's psychology, and particularly in his discussion of the cognition of the soul's substantiality and identity. Most generally, Leibniz contends that these concepts themselves arise through a sort of Lockean reflection on myself, writing that "the notions which I have of myself and of my thoughts, and, consequently of being, of substance, of action, of identity, and of many others, come from an internal experience" (GP IV, 452–3; L 321). More to the point, Leibniz argues that internal sensation and experience provide the necessary (if not independently sufficient) basis for the cognition of the soul's substantial identity. So, Leibniz writes that "I sense myself [je me sente] to be a substance that thinks" (GP II, 45; AG 75) and similarly, regarding the cognition of my own identity over time, Leibniz claims that "my internal experience convinces me a posteriori of this identity" (GP II, 43; AG 73). Significantly, Leibniz also emphasizes the limits of this empirical cognition of my own substantiality and identity, claiming that merely sensing that we think amounts to a "confused experience," falling short of a distinct conception of "what distinguishes me from all other minds" (GP II, 45; AG 75). Even so, Leibniz contends that this initial experience serves as an indispensable starting point on the basis of which we proceed to search for "an a priori reason" that accounts for the distinct cognition of our individual identity. Accordingly, while Leibniz no doubt emphasizes the limitations of our a posteriori cognition when it comes to disclosing the soul's substantiality and 9 identity, what is also clear is that Leibniz takes the application of reason as following upon this empirical basis. Much as was the case with theology or astronomy, Leibniz thus appears to conceive of experience and reason as forming two parts of a single continuous investigation of the nature of the soul.7 The result of Leibniz's admission of empirical cognition into psychology is that it ultimately brings the methodology of psychology closer to that of physics, and Leibniz is particularly explicit in his New Essays that his psychology will take its methodological cue from natural science. Indeed, and again in stark contrast with Descartes, Leibniz emphasizes the parallelism between physics and psychology, claiming for instance that his doctrine of insensible perceptions is "as important to pneumatology as insensible corpuscles are to natural science" (NE 56; cf. also 116). Significantly, this metaphysically-grounded assumption of the parallelism between the investigations of corporeal and thinking nature drives Leibniz's psychological inquiries in key instances, as Leibniz allows that we can make conjectures regarding the immaterial world on the basis of the more familiar observations of such beings in the material world (NE 389; cf. also 490).8 A ready example is provided by Leibniz's criticism of the Lockean denial that the mind always thinks; so, while Leibniz admits that souls are often without thought, that is, consciously-had perceptions, he claims that the soul is never without unconscious perceptions, and in support of this he makes use of an analogy with the observed motions of bodies: "I maintain that in the natural course of things no substance can lack activity, and indeed that there is never a body without movement" (NE 53). Perhaps surprisingly, the obvious parallels between Wolff and Leibniz can in this case plausibly be traced to Leibniz's influence, as it was Leibniz who, near the outset of his correspondence with Wolff, urged him to pursue the analogy between physics and psychology which Wolff had begun to explore in his dissertation on universal practical philosophy.9 7 Contrast, for instance, Wilson, who identifies these as "conflicting tendencies in the treatment of self-consciousness" (see "Leibniz: Self-Consciousness and Immortality", in: Archiv für Geschichte der Philosophie, (1976), 335-52; here 345). 8 See also the "Reply" to Bayle's note 'L' to the article "Rorarius" in the second edition of the Dictionary: "the reason for the change of thoughts in the soul is the same as that of the change of things in the universe which it represents" (GP IV, 562; L 579). 9 See Leibniz's letter to Wolff of Feb. 21, 1705 (BLW 17–18), and Robert J. Richards: "Christian Wolff's Prolegomena to Empirical and Rational Psychology: Translation and Commentary", in: Proceedings of the American Philosophical Society, 124(3) (1980), 227–39; cf. 229n19. Given this, Engfer's claim that Wolff's psychology represents a departure from Leibniz's simply because Leibniz distinguishes between the (causal) order of nature and the (teleological) order of spirits is too hasty (see Engfer, "Von der leibnizischen Monadologie zur empirischen Psychologie Wolffs," and "Konzeption des Psychischen und der Psychologie zwischen Leibniz und Wolff", in: Wegbereiter der Historischen Psychologie, G. Jüttemann (ed.) Munich/Weinheim, 1988; 23-7). 10 In the foregoing, then, I have argued that a consideration of Leibniz's scattered discussions of the human soul's nature, powers, and immortality in light of Wolff's treatment reveals the outlines of a much more complex approach to the soul, one that consistently draws on empirical observation and proceeds in a manner consistent with scientific method in order to arrive at grounded cognition of the soul. By way of conclusion, I will only briefly note the significance of the evidently Wolffian character of Leibniz's psychology. While it might not initially seem terribly surprising given Leibniz's acknowledged influence on Wolff that Leibniz and Wolff agree on the general character of the investigation of the soul, yet many of the texts in which Leibniz elaborates this position most clearly, such as the New Essays or the correspondence with Arnauld, would not have been among those available to Wolff when he set to work on his metaphysics. Accordingly, rather than resulting from a direct influence of one thinker upon the other, it is likelier that Leibniz's and Wolff's psychology are reflective of a broader sort of rationalism that was championed in this period, one namely that did not reject the relevance of experience and observation for metaphysical questions and which self-consciously set out to adapt a scientific method that had seen enormous success in explaining corporeal nature for application to thinking nature. Interestingly, the broader rationalistic character of Leibnizian-Wolffian psychology was not lost on the German thinkers after Wolff, particularly Kant, who was the first to recognize that for any rational psychology that purported to take the merely formal I think as its "sole text," any empirical admixture would corrupt the purity of that putative science. As Kant writes in his devastating criticism of this psychology in the Paralogisms, "if we also made use of observations about the play of our thoughts and the natural laws of the thinking self" in the context of a rational investigation of the soul, then we could never claim to "teach apodictically about thinking beings in general something touching on their nature" and thus that our doctrine of the soul "would be no rational psychology" (A347/B405-6).