1.1. Wolff on the powers of the soul

It might seem that the first place to look among Kant’s historical predecessors when discussing the historical and dialectical background of Kant’s discussion of the powers of the soul and the simplicity of the soul would be Baumgarten’s Metaphysica (Metaphysics) (1739) given that Kant lectured on the basis of this textbook throughout his career.Footnote ⁶ However, Wolff rather than Baumgarten or any other German rationalist was really at the center of debates about the number of the soul’s fundamental powers and the associated debates about the soul’s simplicity. In his discussions of the soul and its powers, Wolff relies on his views on the nature of thought established in his Deutsche Metaphysik (German Metaphysics) (1720).Footnote ⁷ According to Wolff, thinking, which he equates with consciousness, consists in the capacity to cognize the ‘difference between the soul and those things that are represented’ (Wolff [Reference Wolff1720] 1751, §730, §729), which also requires the capacity to distinguish between oneself and objects external to oneself and to differentiate among the various objects presented in conscious thought. He writes, for example: ‘we find, accordingly, that we are conscious of things when we differentiate them from one another’ (§729). We also become conscious of our thoughts of ourselves ‘when we notice the difference between ourselves and other things of which we are conscious’ (§730). The capacity to distinguish between oneself and the objects of thought and between the various objects of thought also requires additional capacities. According to Wolff, ‘[i]f one wishes to distinguish things from one another, one must compare them’ (§733). Comparison also requires the capacity to retain thoughts in memory: ‘When one compares the thoughts, one must not only retain what is thought but also know that one has already had these thoughts and so must have a capacity for memory’ (§734).Footnote ⁸ So for Wolff, thought requires certain mental capacities, which include the capacity to retain thoughts, reflect on them, compare them, and synthesize them into unified representations (§730, §733, §734, §735), all of which ‘is an activity of the soul’ (§730). Because ‘a capacity is only a possibility of doing something’ (§117), these mental capacities are not sufficient for thought, so they must also be grounded in an actual power of the soul, which is an activity that is the source of capacities and actual changes.

Wolff argues furthermore that the capacities associated with thought must be grounded in a single substance that possesses a single fundamental power of representation or vis repraesentativa.Footnote ⁹ Although it may appear that the soul has several powers such as memory, imagination, and volition, Wolff argues that ‘a plurality of powers distinct from each other cannot be found in the soul, because otherwise every power would require a self-subsisting thing to which it would be ascribed’ (§745). The fact that we appear to ourselves as having various powers is due to the fact that we are able to distinguish conceptually between these powers, but this does not entail that these powers are also in fact distinct at a more basic ontological level (§745). A plurality of distinct powers cannot be found in the soul because, according to Wolff’s ontology, each power must be grounded in a distinct self-subsisting simple substance. And if the soul was or had several powers, then each power would need to be grounded in a distinct self-subsisting simple substance, which would make the soul a composite of such substances. Wolff makes a similar argument very clearly in the Psychologia rationalis (1734) (Wolff Reference Wolff, Wolff and École1734), where he writes:

Wolff’s point in the Psychologia rationalis is that each power requires a distinct and independent substance, and if the soul had a plurality of powers, each of these would require a distinct substance, which would entail that the soul is a composite. He is quite clear throughout his writings that it is absurd for the soul to be composite because as such it would be subject to the dissolution of its parts and would therefore not be immortal. But more can be said here about the sense in which a soul endowed with multiple powers would be a composite of substances each endowed with a single power.

According to Wolff in the Deutsche Metaphysik, we call ‘that which consists of many parts that are distinct from each other, but that follow upon each other in a certain order and are connected with each other, a composite thing’ (Wolff [Reference Wolff1720] 1751, §51).Footnote ¹⁰ A composite thing necessarily fills space since it consists of distinct parts existing external to and adjacent to one another (§52). According to Wolff, we represent coexisting distinct things as being external to one another:

The distinct parts Wolff refers to here are simple substances. Since simple coexisting substances are distinct from one another, we represent them as external to one another. And when they are aggregated into a composite such simple substances fill space:

Distinct coexisting simple substances are external to one another and stand in an ordered relationship with one another; they are on top of or adjacent to one another, for example. Because space for Wolff is nothing other than this ordered relationship among simple substances, a composite of substances can be said to fill space. And it can do so despite the fact that individual simple substances themselves do not actually fill space, but are only points in space.Footnote ¹¹ Additionally, since simple substances cannot coexist in one point and each simple substance requires its own point, many taken together constitute a single composite that acquires extension:

As Eric Watkins points out, for Wolff, the simple constituents of composites are points, but they are not mathematical points because the simple constituents are distinct from one another, and mathematical points are identical (Watkins Reference Watkins, Garber and Nadler2006, 282).Footnote ¹² Since simple substances are distinct in this way, Wolff believes aggregates of these substances acquire extension.

Surprisingly, however, it appears to follow from Wolff’s account of composites not only that simple substances are necessary for constituting a spatial, extended composite but also that they are sufficient. This is to say that whenever distinct simple substances are aggregated, they form an extended composite. This is, however, not an innocuous consequence of his account. For example, it seems to rule out the possibility that there could be a composite of immaterial souls that is not an extended composite, such as a kingdom of spirits working together. Although a composite of souls might exist and might on Wolff’s account ground accidents other than extension, such as some sort of collective action, it appears that the composite of souls would also necessarily be extended. Since simple substances are each points, and these points must be external to one another, a composite of such substances would necessarily be extended. Given Wolff’s view that a simple substance can possess only a single fundamental mental power, and his account of how aggregates of simple substances constitute extended composites, it also follows from Wolff’s account that a soul endowed with multiple fundamental powers would be an extended composite. The fact that Wolff thinks of a composite soul as a spatial, extended composite is evident throughout the Deutsche Metaphysik where he equates a composite soul with matter and argues that the soul could not be an extended composite, or matter, because such a composite would be incapable of the kind of thought we associate with the soul (Wolff [Reference Wolff1720] 1751, §738). Although one might wonder whether Wolff would ultimately have endorsed the idea that a soul endowed with multiple fundamental powers would necessarily be an extended composite of distinct substances each endowed with distinct fundamental powers, it does appear to follow from his account of composition and is in keeping with his discussions of the soul as an extended composite.

What I have provided thus far is a realist interpretation of Wolff’s views on the relationship between simples and extended, spatial composites according to which simples actually constitute extended, spatial composites. This interpretation also resonates with Watkins’ interpretation. However, in Cosmologia generalis §144, Wolff writes: ‘… extension is a phenomenon in the same sense in which color is accustomed to be called a phenomenon ….’ (Wolff Reference Wolff and Wolff1737). Such passages have led Lewis White Beck to argue that for Wolff extension and space are due to the confused representations of perceiving subjects and that Wolff therefore maintained a ‘subjectivistic’ theory of space (Beck Reference Beck1969, 269). At the same time, Beck also acknowledges the realist elements in Wolff’s account. In contrast with Beck, on my interpretation, however, the extension and spatiality of composites is due to the nature of the simple substances that ground these composites rather than to perceiving subjects. There is certainly evidence for both interpretations in Wolff, and in many ways Wolff’s ambivalence is not surprising given that it reflects a similar ambivalence in Leibniz’s conception of the relationship between monads and extended composites. However, I think the strongest case for thinking that Wolff maintained the realist view is that he appears to draw consequences from it which could not be drawn if he maintained the subjectivist understanding of space. As we have seen, Wolff enlists his understanding of how extended, spatial composites arise from the real aggregation of simples to argue that a soul endowed with multiple powers would be an extended composite. And Wolff’s argument would not work if extension were the result of merely confused representations of simples because it would just show that we confusedly represent a soul endowed with multiple mental powers as an extended composite. If this were the case, the soul would not really be subject to the dissolution of its parts. It also becomes clear, as we will see, that Crusius and others understood Wolff to be maintaining the realist view of the aggregation of simples into extended, spatial composites.

It should be evident then from the preceding discussion that one reason Wolff would reject the idea of a soul endowed with multiple fundamental powers is that such a soul would be an extended composite of distinct individual substances each endowed with a distinct fundamental power. But he also provides an additional argument against a soul endowed with multiple fundamental powers. According to Wolff, a power consists in a striving to do something, in an activity. And if a soul consisted in several such strivings, it would be pulled in different directions ‘as if a body, which is to be viewed in its motion as an indivisible thing (§667), should move in different directions at the same time, ’ which is absurd (Wolff [Reference Wolff1720] 1751, §745).Footnote ¹³ Such a soul would lack the kind of unity of activity characteristic of a soul, which it can have only if it possesses a single fundamental power. And, although Wolff does not say this explicitly, it follows from his earlier considerations that this unity of activity must be attributed to a single simple substance. Unfortunately, Wolff is not explicit in these passages about what exactly the unity of activity of a soul would consist in. But his follower, Knutzen, is much more explicit about what this unity consists in and why it requires a single fundamental power. In his Philosophische Abhandlung von der immateriellen Natur der Seele [Philosophical Treatise on the Immaterial Nature of the Soul] (Reference Knutzen1744), Knutzen agrees with the Wolffian idea that consciousness requires the capacity to distinguish oneself from other things and argues that in order to do this the soul must compare, contrast, and synthesize representations on the basis of a single ‘efficacious power’ (Knutzen Reference Knutzen1744, §2–4, §6). A composite whose parts each possessed its own power could not provide for the kind of unity of synthesized representations the soul has as a thinking being.

If this interpretation of Wolff is correct, we have seen that he rejects multiple fundamental powers for at least two reasons: they would entail an extended, composite soul, and they would not allow for the unity of activity characteristic of thought. Wolff’s claim that a soul could possess only a single fundamental power and the conception of substances, powers, and composites upon which he bases his conclusion had a number of supporters such as Knutzen, but it also met with fierce criticism from Crusius and others. Before considering Kant’s response to Wolff’s claims, it will be worthwhile to consider Crusius’ response to Wolff’s rejection of fundamental mental powers.

1.2. Crusius on the powers of the soul

In his Entwurf der nothwendigen Vernunft-Wahrheiten (Sketch of the Necessary Truths of Reason) (1745), Crusius argues in favor of the idea that the soul can have multiple fundamental powers without being a composite. Crusius understands fundamental powers as follows. For Crusius, a power in the broad sense is the ‘possibility of one thing attached to another thing’ (Crusius [Reference Crusius1745] 1766, §63). This can be illustrated with an example. Consider two substances A and B. The property in A whereby it causes some other substance B to have a property is a power (§63). When a person A throws a ball B, this ball acquires the property of being in motion. And it does so in virtue of a motive power in A. This definition also applies to single substances. A soul has some particular thought as a property on the basis of a power that causes this thought. This notion of power in the broad sense leads Crusius to the notion of a power in the narrow sense, or what Crusius calls a ‘fundamental power’ (Grundkraft). Given the former definition of a power in the broad sense, we can attribute any number of powers to a thing. The person A might have a specific power that causes the ball B to travel in a specific direction at a specific speed. Or the soul might have a specific power that causes desires, envy, analysis, synthesis and so on. In this case, various effects are simply subsumed under some distinct power. However, this subsumption of an effect under a power is not yet genuinely explanatory according to Crusius because the effect could be the result of a power that has some other power or composite of powers at its basis (§70). The ball B acquiring a certain speed and trajectory could be caused by a single or several combined powers in person A. The soul’s desirous or analytic thoughts could be caused by a single or several combined powers. As Crusius argues, however, in order genuinely to explain the occurrence of an effect, we have to look not just at powers but at the fundamental power or powers that lie at the basis of this effect. According to Crusius, since all effects that are attributed to things arise from their fundamental essence or the fundamental essence of other things (§39), we have to determine what powers constitute the fundamental essence of a thing (§70). These powers that determine the essence of a thing and explain the effects that this thing can cause are its fundamental powers.

In the course of his discussion, Crusius identifies eight different characteristics of fundamental powers. And several of these features figure prominently in his discussion of fundamental powers of the soul: (1) ‘A finite fundamental power constantly has one and the same proximate effect, and more remote effects must be comprehended through it’ (§73). This is also to say that if a power appears to have different proximate effects, it is not a fundamental power. It may, for example, be a composite of powers whose proximate effect differs depending on external circumstances. (2) ‘The conditions by which the action of a fundamental power is to be restricted must lie in the very same subject’ (§74). If the power resided in another subject, the effect through which one thinks the power could either not be understood at all on the basis of this power, or it would have to be understood on the basis of its own power and the power of the other substance, which means it would be a composite and not a proximate effect. In either case, one would not have identified a fundamental power. (3) ‘Nothing can occur in the effect for which a power cannot be found in what one posits as its sufficient cause’ (§78). For example, the will and the power of representation must be distinct fundamental powers because although desire presupposes representations, it cannot be understood on the basis of representations alone. There is something in the will that the power of representation does not have the power to cause, so they must be distinct fundamental powers.

For Crusius, the understanding of a rational and finite spirit is not a fundamental power but a plurality of fundamental powers. And the powers and capacities derived from these fundamental powers are modes of thought that contribute to our pursuit of knowledge (Crusius [Reference Crusius1745] 1766, §444, 907). In his description of the powers of the mind, Crusius rejects the claim that the soul possesses only a single fundamental power on several grounds. One argument is this: Our ideas and mental states exhibit a great deal of qualitative difference. If, however, one thinks as Wolff does that each of these mental states is grounded in a single fundamental power, then a problem arises. Since each idea is qualitatively different, it shows that the fundamental power does not consistently produce similar effects. But this seems contrary to what one would expect of a fundamental power. One would expect that a fundamental power would consistently produce similar effects. In Crusius’ words: ‘I conclude, the proximate actions of a fundamental power would not consistently be similar, which must be the case if all ideas were activities of a single fundamental power’ (§444, 909). And the fact that one needs to account for the variety and qualitative difference of ideas might then lead one to think that they must be grounded in a plurality of fundamental powers. Although Crusius admits that not every idea would require a fundamental power, since some ideas are constructed out of other ideas, he nevertheless suggests there is good reason to think that the soul is endowed with more than one fundamental power. A second argument is this: Consciousness of the action whereby an object is represented adds something that is not in the original representation of the object. Through consciousness, we have a representation of our thoughts. And just as the object and the representation of this object are not the same thing, the consciousness of a representation and this representation are not the same thing. If this is the case, then we must admit that consciousness requires a special fundamental power distinct from the power of representation: ‘One must therefore admit that consciousness requires a special fundamental power through which it is possible’ (§444, 910). This of course contrasts with those who believe that consciousness is only a degree of the power of representation rather than a distinct fundamental power as well as those who believe that consciousness arises from our distinguishing concepts (§444, 910). We have also seen that Crusius makes a similar argument to show that the will and the power of representation must, contra Wolff and Leibniz, be distinct fundamental powers (§78).

Crusius also argues against the central Wolffian thesis that a soul endowed with multiple fundamental powers would be a spatial, extended composite. He writes:

According to Crusius, one need not conclude that the soul is a spatial, extended composite from the fact that it possesses multiple fundamental powers. This is because one need not think as Wolff does that each distinct power requires a distinct substance, or that these powers and the individual substances that ground them are external to one another and spatially ordered.Footnote ¹⁴ Crusius suggests that what follows from the assumption of multiple fundamental powers in the soul is not a spatial, extended composite, but a plurality of action or activity. One substance can produce a variety of actions through its plurality of fundamental powers. The soul, for example, can both represent things to itself and desire those things or not, will them or not. This means that the soul can possess multiple fundamental powers and remain a simple substance.Footnote ¹⁵ And although Crusius never explicitly attacks the Wolffian claim that a soul endowed with multiple fundamental powers would not have the requisite unity of activity, it is clear he rejects this idea insofar as he thinks that multiple fundamental mental powers are involved in thought.

Kant was also well aware of these debates regarding the fundamental powers of the soul. In the following section, I consider Kant’s idea in the Subjective Deduction that the mind has multiple fundamental mental powers and how aspects of the Second Paralogism and his lectures on metaphysics can be used to defend this view against Wolff’s arguments against the existence of multiple fundamental mental powers.

2.1. Powers and the Transcendental Deduction

In the previous section, we saw that the discussion of the number of fundamental powers that could be attributed to the soul was intimately connected with debates about the nature of substances and the simplicity or compositeness of the soul. Kant’s multiple statements throughout the Critique of Pure Reason and his lectures on metaphysics indicate that he was well aware of the connections between questions about the number of fundamental powers and the nature of the soul that grounds these powers.Footnote ¹⁶ Kant also takes up the issue of fundamental mental powers explicitly in the Subjective Deduction. In the preface to the A edition of the Critique of Pure Reason, Kant discusses the objective and subjective sides of the Transcendental Deduction. Regarding the Subjective Deduction, he says: ‘[It] deals with the pure understanding itself, concerning its possibility and the powers of cognition on which it itself rests’ (A xvii). Its purpose is to consider ‘How is the faculty of thinking itself possible?’ which Kant glosses as ‘something like the search for the cause of a given effect’ (A xvii). Whereas the Objective Deduction seeks to clarify the objective validity of the concepts of the understanding a priori, the Subjective Deduction investigates the understanding, which Kant equates with the faculty of thinking here, as the effect of certain ‘powers of cognition.’ As we have seen, this is also precisely how Wolff raised the question of a fundamental power, by considering the faculties and powers that are required in order for the unity of the soul’s activity of thinking to be possible. And Kant’s construal of the search for mental powers as something like the search for the cause of a given effect is also reminiscent of Crusius’ understanding of the search for fundamental powers. Kant’s aim in the Subjective Deduction is to provide an account of the various powers that give rise to or cause our capacity for thinking. In contrast with Wolff, however, who maintains that all powers of thought that we exhibit are grounded in a single representative power (vis repraesentativa), Kant argues that our mental capacities, particularly sensibility and understanding, cannot be reduced to a common cause or single fundamental power and that the mental powers he identifies in the Subjective Deduction are distinct and jointly necessary for cognition.

In a paragraph omitted from the second edition of the Critique, Kant writes the following regarding our distinct mental powers which are jointly necessary for cognition and cannot be derived from some more fundamental mental power:

Unlike Crusius, who rejects Wolff’s account of consciousness, Kant does not appear in the Critique of Pure Reason to disagree with Wolff’s idea that thought requires the capacity to distinguish, compare, and synthesize representations. Indeed, Kant sets out three kinds of synthesis – of the manifold through sense, through the imagination, and the unity of this synthesis through apperception – that contribute to the possibility of experience, by which Kant means the unity exhibited in our thinking. Although Kant and Wolff differ in the details about how to classify these kinds of synthesis, they are in broad agreement, for example, that representations must be combined and that this requires some kind of retention of representations. However, rather than suggesting that such forms of synthesis can be reduced to a single power of representation, Kant argues that these forms of synthesis have

This is to say that Kant identifies some effect, the unity of our thinking, and suggests that this effect has its source in three fundamental faculties that are distinct and cannot be derived from some more fundamental faculty. Although Kant mentions only ‘faculties’ here, it should be clear from his description of the aims of the Subjective Deduction as a search for certain ‘powers of cognition’ and his interchangeable use of ‘faculties’ and ‘powers’ throughout his writings that this discussion is really about fundamental mental powers.Footnote ¹⁷

It is not important for our purposes here to uncover the details of how Kant argues that these three transcendental faculties or powers are irreducible to a single fundamental power and are jointly necessary for thinking.Footnote ¹⁸ Suffice it to say that Kant identifies certain empirical capacities that are used in cognition and proposes that each has a necessary transcendental ground. Nor is it important whether the irreducible powers Kant identifies in the Subjective Deduction represent his considered view on which powers exactly are fundamental.Footnote ¹⁹ It is only important to note that he identifies multiple fundamental powers and that regardless of whether his argument is convincing or not, there is a deep component of the historical discussion of mental powers that is in part left out of Kant’s discussion in the Subjective Deduction, namely the issue of the substance in which the powers reside. As can be seen from the passage above, Kant suggests that the ‘three original sources, ’ ‘which contain the conditions of the possibility of all experience, ’ are ‘capacities or faculties of the soul’ (A 94). Although Kant’s wording here regarding a soul might appear to be merely a façon de parler, it is not. Rather, it suggests that although Kant explicitly disagreed in the Subjective Deduction with the origin of our mental powers in a single fundamental power, he nevertheless recognized that the discussion of mental powers was intimately tied to discussions about the powers of the soul.Footnote ²⁰ Footnote ²¹ Nor is this surprising given Kant’s familiarity with the debates about the powers of the soul. Kant’s allusion to the soul also reflects his well-known ambivalence throughout the A edition regarding the substantiality of the soul.Footnote ²² Neither is it surprising, however, that Kant does not engage explicitly with the debate about whether a soul could possess multiple fundamental powers in the Subjective Deduction since the focus of the Transcendental Deduction as a whole and the entire Analytic of Concepts in the Transcendental Analytic is not primarily with the metaphysical views of his predecessors but attempts as much as possible to bracket such discussions in order to develop an analysis of cognition. In order to see whether Kant is in a position to answer Wolff’s objections to the attribution of multiple fundamental powers to the soul, we have to turn to Kant’s discussion of the soul and its compositeness in the Second Paralogism and his lectures on metaphysics.

2.2. Unity of thought and the simplicity of the soul

Recall that in the foregoing discussion we saw that Wolff argues that a soul endowed with multiple mental powers could not have the unity of activity required for the soul. And we have also seen that the Wolffian philosopher Knutzen expands upon this insight by arguing that the cognitive unity of the soul requires that the soul be able to compare and synthesize thoughts and that only a single fundamental mental power could be responsible for this cognitive unity. We have further seen that Wolff maintains that each fundamental power must inhere in a distinct simple substance. And ultimately this means that the unity of the activity of the soul entails that the soul is a simple substance endowed with a single fundamental power. In the Second Paralogism, Kant agrees with what is essentially a Wolffian idea made more explicit by Knutzen, namely that ‘we demand absolute unity for the subject of thought’ (A 354). And as the Transcendental Deduction makes clear, this means that our mental powers must synthesize representations into a coherent cognitive unity.Footnote ²³ Kant, however, argues that it does not follow from the unity of thinking that the ground of this unity must be a simple soul rather than a composite. We may consider Kant’s argument before considering why this is important for Kant’s discussion of multiple fundamental mental powers.

In his discussion in the Second Paralogism of the idea that the unity of thinking entails the simplicity of the soul, Kant first considers the idea that a composite of substances cannot produce the unity of thinking. In his explication of the argument for this thesis, Kant first considers what a composite is and then distinguishes between a composite that produces an external effect as an accident and a composite that produces an internal effect as an accident.Footnote ²⁴

In his discussion, Kant considers whether an effect can inhere in a composite. But given the understanding of inherence common to the period, and his statements elsewhere in the Second Paralogism, it is clear that the question is whether an effect can have its causal ground in a composite. According to Kant, the rationalist recognizes that a composite of substances may causally ground a unified effect when this effect is merely external. This is the case with any physical body, where its overall movement is grounded in the movement of each of its parts. For example, the parts of a human body, its legs, arms, and so on are each a substance that produces an action through its power: the muscles of the legs become tense and release, the arms move forward and backward. And these individual actions each combine to produce the effect or activity of walking. Using Kant’s terminology of accidents, walking can also be an accident of a composite, as in the statement ‘the man is walking, ’ where the accident ‘walking’ is attributed to the composite substance ‘man.’ The rationalist denies, however, that this is also the case with a thinking being or soul, where the effect, thinking, is internal rather than external. Kant illustrates this point using the analogy with a verse.Footnote ²⁵ Imagine that the individual words of a verse were divided among several individuals. If this were the case, the individual words of the verse would not constitute a whole verse. As we have seen, for example, Knutzen argues that the various parts of a thought could not be synthesized into unified thought unless this synthesis were grounded in the efficacious power of a single substance. Similarly, Kant’s rationalist argues that ‘the representations that are divided’ among a composite cannot ‘constitute a whole thought’ because the unity required for thought cannot be causally grounded in a composite. And since the thoughts cannot have their causal ground in a composite, they must have a causal ground in an ‘absolutely simple’ substance.

According to Kant, the rationalist claim that a unified thought has to be considered as the effect of a simple substance is neither a legitimate a posteriori nor synthetic a priori claim. It cannot be a legitimate a posteriori claim because knowledge of both the ‘absolute unity’ of a thinking being and the necessity involved in the rationalist’s claim exceeds the bounds of experience (A 353). It also cannot be a synthetic a priori claim because the proposition does not express or derive from a necessary condition for the possibility of experience (A 353).Footnote ²⁶ This is to say, there is no reason to think that it is a necessary condition for the possibility of experience that a unified thought must be the effect of or causally grounded in a simple substance.Footnote ²⁷ Kant also attacks the claim by arguing that the analysis of the notion of a unified thought does not necessarily entail that a simple substance must be the causal ground of the unity of thought, i.e. that it is not a necessary truth that the unity of thought cannot be produced by a composite of substances. He writes:

Similar to the example of the walking man, Kant maintains that it is logically possible that the unity of thought as an internal attribute is causally grounded in a composite of distinct substances acting together to produce the unity of thought. One might think, for example, of the statement ‘the man is thinking.’ Since there is nothing incoherent or contradictory about this idea, it cannot be an analytic a priori or necessary truth that thought cannot be grounded in a composite. Likewise, if it is possible that thought can be grounded in a composite of substances, then it is not necessary that thought cannot be grounded in a composite of substances.Footnote ²⁸ However, although Kant points out this possibility, he is reticent about how such a possibility could be explained. But it is quite open to Kant to argue that it is logically possible that the unity of thought is an emergent property that has its ground in a composite of substances. Kant was likely well aware of the postulation of emergent properties in chemistry by seventeenth- and eighteenth-century scientists, so it would not be anachronistic to think that Kant may have had emergent properties in mind when he dismisses the rationalist’s argument.

Where does this interpretation of Kant’s rejection of the rationalist’s argument stand with respect to other recent interpretations? Corey Dyck has recently recognized that Kant’s discussion here is very much concerned with the question of mental powers as the causal grounds of thoughts (Dyck Reference Dyck2014, 132–5). For the Wolffian, what it means for an accident to inhere in a substance is for it to be the result of the activity of a substance. This is why Kant equates the rationalists claim that ‘a thought can be only the effect of the absolute unity of a thinking being’ with the idea that the unity of thought could not inhere in a composite. However, in contrast with the interpretation above, Dyck argues that Kant’s main aim is to reject the rationalist’s claim that any substance or composite of substances could be the causal ground of thought, i.e. that the unity of thought could be the effect of a substance or substances. The rationalist’s causal claim is neither analytic, because it is a causal claim, nor synthetic a priori, since it is not a condition of possible experience, nor a posteriori, since experience is not sufficient to establish necessity. This reading seems problematic, however, insofar as Kant is not explicit about attacking the causal claim as such but rather appears, as can be seen from the quotations above, to be concerned with whether the unity of thought could be causally grounded in a composite of substances or whether this causal ground must be simple (A 353). Other interpretations, such as that provided by Colin Marshall, also rely on the idea that Kant is suggesting that it cannot be ruled out that multiple substances could be the causal ground of the unity of thought (Marshall Reference Marshall2010, 15).Footnote ²⁹ Moreover, there is the broader issue that Kant does in fact appear to maintain elsewhere that the soul can and must be a causal ground of thought and that we can know this. It is central to his view of freedom, for example, that the intelligible self has to be a free cause of the choice of maxims from which its empirical actions flow. However, it can be conceded that although it appears clear that Kant maintains such causal claims, it is unclear how exactly this is supposed to fit with his claim that the category of causation does not apply to things in themselves. It may be that Kant might have in mind some kind of noumenal, atemporal sense of causation; however, a discussion of Kant’s views on noumenal causation would take us beyond the scope of this paper.Footnote ³⁰

Notwithstanding these questions about causation, it should be clear from the foregoing discussion that Kant thinks that the rationalist’s argument that the unity of thought requires a simple soul is dubious. But what does this reveal about Kant’s discussion of multiple fundamental mental powers? As we have seen from the discussion of Wolff, the unity of the activity of thinking requires a single power and a single power must be grounded in a single simple substance. Likewise, the unity of thought for Knutzen requires a single efficacious power grounded in a single simple substance. Given the discussion above we can see, however, that even if Kant were to accept the premise that each power must inhere in a single simple substance, he does not think that the unity of thought entails the simplicity of the soul. This is to say that the unity of thought could be the result of a composite of substances each endowed with a single fundamental power working together to produce the unity of thought as an effect. But this is not to say, however, that Kant is providing a positive argument in the Second Paralogism that the unity of thought is in fact grounded in a composite of substances each endowed with powers. His argument just shows that the unity of thought is not a decisive reason to reject the idea that a composite of substances could possibly ground this unity nor, by extension, that a composite of substances each endowed with a single power could produce such a unity.

2.3. Powers, extended composites, and the soul

We have seen that Kant argues that a composite could ground the unity of thought contrary to what the rationalist has argued. This suggests that even if multiple mental powers entailed a composite of substances this would not undermine the unity characteristic of the activity of thinking. Recall, however, that Wolff also rejects multiple mental powers because a soul endowed with multiple mental powers would be a spatial, extended composite of simple substances each endowed with a single power. The Second Paralogism also contains a hidden argument against this conclusion. At A 356–361, Kant points out that the sole reason the rationalist wishes to establish that the soul is simple is in order to distinguish it from matter, which is an extended composite. The motivation here is that if the soul can be shown to be non-composite, then it entails that it cannot perish through the dissolution of its parts. As Kant writes: ‘[T]he assertion of the simple nature of the soul is of unique value only insofar as through it I distinguish this subject from all matter, and consequently except it from the perishability to which matter is always subjected’ (A 356). This assessment is also reflective of Wolff’s discussion of the simplicity of the soul as we have seen. However, according to Kant, properties associated with matter such as extension, spatial compositeness, and motion are properties only of appearances of outer sense and not of things in themselves.Footnote ³¹ So the thing in itself or things in themselves that ground the unity of thought could not be an extended composite. As he writes: ‘But this Something is not extended, not impenetrable, not composite, because these predicates pertain only to sensibility and its intuition, insofar as we are affected by such objects (otherwise unknown to us)’ (A 358).Footnote ³²

Kant’s claim that only an appearance and not a thing in itself could be an extended composite offers an interesting way to build upon Crusius’ recognition that a substance with multiple fundamental powers need not be an extended composite as Wolff argues. Kant may argue against Wolff that a soul endowed with multiple fundamental mental powers would not be an extended composite of simple substances each endowed with a fundamental power because the soul is a thing in itself. And as a thing in itself it cannot be an extended composite. This is to say that even if it were true that each power required a distinct substance, an aggregate of such substances would not be a spatial, extended composite since spatiality and extension apply only to appearances and not things in themselves. Given this view of extension, Kant is also in a position to support his view from the Subjective Deduction that we are endowed with multiple fundamental mental powers against Wolff’s argument that such a soul would necessarily be an extended composite. Regardless of whether multiple fundamental powers would require multiple substances, this does not mean that a soul endowed with multiple fundamental powers would be a spatial, extended composite. This is not, however, to say that Kant accepts the Wolffian premise that each fundamental power requires a distinct substance, nor is it to say that Kant is suggesting that things in themselves could not be composite simpliciter. Rather, the point is just that a composite of noumenal substances would not be a spatial, extended composite, and therefore in attributing multiple fundamental mental powers to the soul, Kant’s position would not be subject to the problems Wolff raises.

Of course, a number of objections might be raised here. For one thing, it may be objected that attributing such an argument to Kant relies on a metaphysical interpretation of transcendental idealism according to which things in themselves lack spatial properties. This contrasts with epistemological or methodological interpretations that might hold that things in themselves are merely considered independent of the spatial form of intuition. However, the former interpretation has a great deal of evidence in Kant’s text and has been well defended in the literature, and given the constraints of this paper it will simply have to be assumed as a premise in the argument. For another thing, Wolff might also attack the idea that space and extension are not properties of things in themselves. Kant, however, goes to great lengths to establish both of these points in the Critique of Pure Reason. And it should not be surprising that the success of the argument against Wolff will depend on the success of Kant’s transcendental idealism as a whole. But regardless of whether the argument is successful we can see that it is a potential response on Kant’s part to Wolff’s argument against multiple fundamental mental powers, which is in keeping with transcendental idealism.

We have seen that Kant is in a position to reject the idea that the unity of thought requires a simple soul and the idea that a soul endowed with multiple fundamental mental powers would be an extended composite. But Kant also explicitly rejects the foundational premise upon which the Wolffian arguments are based, namely the idea that a simple substance of any sort can have only a single fundamental power. Much like Crusius, Kant is skeptical of the a priori arguments for the claim that a substance may possess only one power. Throughout his lectures on metaphysics, Kant expresses skepticism of the Wolffian view. In Metaphysik Herder (1762–1764), for example, he is reported as saying:

And he quite explicitly criticizes the Wolffians when he writes:

In the first quotation, Kant argues that a soul endowed with multiple fundamental powers need not be a composite since powers are accidents of substances and substances can have many accidents and thus many powers. And in the second quotation, Kant argues that a simple soul can have multiple powers because a power is a respectus. As Kant says elsewhere: ‘power is the relation <respectus> of the substance to the accidents, insofar as it contains the ground of their actuality’ (AA 29:771). So thoughts, for example, may be accidents of a substance, which the substance has in virtue of some mental power. And this power is the relation between the substance and the accidents. At first sight, Kant’s claims in the two quotes above might seem to run counter to one another. In the first, he seems to think of power as an accident and in the second it is a relation. However, they are not so different. Although he is not explicit about this, respectus are just a particular kind of property just as an accident is a property. A respectus is just a property that is instantiated by both the substance and accident. And just as with any other property, such as an accident, a substance can have more than one property. So in this regard, Kant’s claims that a power is an accident in the first quote and a respectus in the second are not so different insofar as both are properties. Kant also points out that a power as respectus contains the ground of some accident. Here is one way to understand this. A particular thought is an accident of a substance. But the substance has this accident only in virtue of some power that the substance possesses. Understanding and sensibility, for example, may be powers in virtue of which a substance has particular thoughts as its accidents. Kant also goes a step further by pointing out that the respectus of a substance can be ‘as various as the accidents are that cannot be reduced to another’ (AA 28:145). He is not explicit about what he means by the reducibility of accidents to a respectus here. But if a respectus is a ground of an accident, then the reduction of an accident to its respectus likely only means that an accident is shown to have some respectus as its causal ground. And insofar as the accidents have distinct grounds, the substance can be said to possess distinct powers. Given that a respectus is just a property of a substance, there is also no reason to think that a substance could not possess multiple fundamental powers.

It should be clear from the preceding that Kant attributes multiple fundamental mental powers to the soul in the Subjective Deduction and is in a position to provide several arguments against the Wolffian rejection of multiple fundamental mental powers. But why then does Kant suggest in the preface that the faculties of sensibility and understanding ‘may perhaps arise from a common but to us unknown root’ (A 15/B 29)? In the Appendix to the Transcendental Dialectic, ‘On the Regulative Use of the Ideas of Pure Reason, ’ Kant offers some reasons why the search for a single fundamental power is important in metaphysics.Footnote ³³ He reiterates there the findings of the Transcendental Deduction, namely that we appear to ourselves to have a variety of faculties – sensibility, consciousness, imagination, memory, wit, the ability to distinguish, desire, and so on – although it is possible that these faculties may be grounded in a smaller number of faculties such as those identified in the Deduction as sensibility, understanding, and reason (B 676f.). However, the idea of a fundamental power that grounds all of these faculties is for Kant only an idea of reason, which demands absolute totality in the synthesis of conditions. Although Kant does not say this, in a substance endowed with multiple powers, these powers would presumably be mutually conditioning insofar as they work together to endow the substance with the capacities it has and therefore also the mental unity that arises from these capacities. But if such powers are mutually conditioning, then reason may still demand that we go further in our pursuit of an unconditioned ground of conditioned attributes. As Kant writes: ‘The idea of a fundamental power – though logic does not at all ascertain whether there is such a thing – is at least the problem set by a systematic representation of the manifoldness of powers’ (A 649/B 677). We proceed by comparing properties of powers and capacities in order to find what they have in common guided by the idea that there is a common power as their ground until we ‘bring them close to a single radical, i.e. absolutely fundamental, power’ (A 649/B 677). Although we can provide no a posteriori or a priori arguments establishing the existence of such a fundamental power, we may, however, use the idea as a means of organizing our investigation of mental faculties.Footnote ³⁴

In this sense, Kant offers only a diagnosis of the tendency to seek a fundamental power and an affirmation of the usefulness of positing such a power in discussions of the nature of the mind. But from his analysis of cognition in the Subjective Deduction as well as his statements throughout his writings, it should become clear that Kant rejects the idea of a fundamental mental power. In the recent literature, there have been two important attempts to justify Kant’s acceptance of multiple fundamental mental powers. Julian Wuerth primarily provides textual evidence to support his claim that Kant accepts multiple fundamental mental powers and suggests that Kant maintains that in our immediate awareness of ourselves as substances, we are also aware of ourselves as exercising these powers (Reference WuerthWuerth 2016, 189–235). He then shows the importance of these powers for Kant’s understanding of how we can justify actions rationally on the basis of our desires but that such reasons are not enough to make an action morally esteemed. And Stefan Heßbrüggen-Walter has claimed that Kant like Crusius maintains that the fundamental powers of the soul are really distinct and are not merely the result of a theoretical construction. And he attempts to justify Kant’s faculty realism internally by arguing that general logic makes non-experiential knowledge of the structure of our faculty of knowledge possible. This knowledge is not dependent on intuition but claims that causally effective faculties are at the root of particular representations in the sense that mental faculties cause mental changes just as physical faculties cause physical changes (Heßbrüggen-Walter Reference Heßbrüggen-Walter2004, 9–26). Heßbrüggen-Walter himself, however, concedes that this approach fails. There is not adequate space in this article to provide a decisive argument for both how and why Kant justifies his claim that the soul has multiple fundamental mental powers that goes beyond the discussion of the Subjective Deduction. But my tentative suggestion is that ultimately the positing of these powers and the soul in which they are grounded is justified not by merely appealing to our immediate awareness of ourselves as substances exercising various powers, since Kant appears to abandon this kind of awareness around the time of the Critique of Pure Reason, nor by appealing to general logic, but by considering how Kant argues that the presupposition of the existence of the soul is necessary for morality.Footnote ³⁵ This justified presupposition of the existence of a simple soul is also accompanied by the presupposition of a free will, for example, and could be expanded to include a justification of various other fundamental mental powers that are required for cognition and moral agency. One step in the direction of understanding Kant’s overall view on fundamental mental powers has been to show that Kant’s attribution of multiple fundamental mental powers to the soul is able to overcome Wolff’s arguments against multiple fundamental mental powers.