1. Metaphysical Foundations

Part of Kraus’s argument against applying the category of substance to inner sense involves rejecting ‘deflationary accounts of substance in outer sense’ (p. 155), according to which ‘even in outer experience, we have only a “bundle” of intuitions of changing states, which are taken to inhere in a substance only by way of the unifying function of the category of substance’ (p. 155, n. 59). On deflationary readings of outer experience, not having direct intuition of inner substance does not make inner experience different in kind from outer experience.

Kraus rejects deflationary views of outer substance, in part based on a passage from A381 in which Kant says that outer intuition includes ‘something standing and abiding’, but primarily based on a reading of the Metaphysical Foundations of Natural Science (MFNS), in which Kant presents a determinate account of corporeal substance, which he explicitly denies to be possible for psychological substance. The key passage for this part of her argument is Kant’s ‘First Law of Mechanics’ (4: 541–3). There Kant applies the general principle that ‘in all changes of nature no substance either arises or perishes’ to the specific case of ‘what substance shall be in matter’, that is, ‘the substance that is possible only in space’ (4: 541). In that context, Kant explicitly says, ‘that which is considered as object of inner sense … does not consist of parts external to one another; and its parts, therefore, are not substances’ (4: 542), and he notes that because ‘the I … [is] merely a thought … of which … one has no concept’ whereas of ‘matter’ we have a ‘concept of the movable in space’, it follows that ‘the persistence of substance can be proved of the latter [matter], but not of the former [the I]’ (4: 543).

For Kraus, this passage shows that we have a determinate intuition of outer substance, namely, the movable in space, and also that we lack a correlative intuition of inner substance, and thus cannot properly apply the category of substance to inner states:

I interpret the MFNS differently. In particular, while there are important differences between outer and inner experience, the category of substance provides conditions of possibility for both.

Before focusing on the details of the passage, I distinguish three applications of the category of substance:

1. 1. The unschematized category of substance, which is irrelevant here. Like Kraus, I allow ‘the applicability of the pure, unschematized category of substance’ to the ‘I, the general correlate of apperception’, and like Kraus, I don’t see this possibility as opening up the application of the category of substance to empirical self-consciousness. Kant’s allusion to the ‘mere thought’ of the I refers to this general subject of apperception.

2. 2. The schematized category of substance in general, that in all appearances which persists, ‘the substratum of the representation of time itself’ (B226). This category of substance is part of what Kant calls ‘the schema … of nature in general’ (MFNS, 4: 474).

3. 3. Empirical concepts of particular kinds of substances, such as that of matter as the ‘movable in space’ (MFNS, 4: 480) or that of ‘thinking being’ (A682/B710; 4: 470). The empirical concept of substance as matter is part of ‘the schema … of corporeal nature in particular’ (4: 474).

Kraus distinguishes (1) from (2) and (3) (p. 166), but does not distinguish (2) from (3). Once we draw such a distinction, we can see the passage as focusing on what can be known a priori from (3), rather than whether (2) applies to this or that sphere of experience. That is, Kant is characterizing ‘the substance that is possible only in space’ and denying that what is considered an object of inner sense is a substance in that sense, without denying that the object of inner sense is a substance in general.

The MFNS passage supports this reading in four ways. First, Kant’s discussion begins by reiterating the general claim that ‘in all changes of nature no substance either arises or perishes’ and then stating that ‘here it is only shown what substance shall be in matter’ (4: 451, emphasis added). Second, the passage aims to prove the conservation of matter a priori (albeit from an empirical concept), and the point of the Remark is not about whether there are mental substances but about what could be proven a priori about such mental substances from their empirical concept. For matter, but not for the soul, one can prove ‘persistence of substance … from its [empirical] concept – namely, that it is the movable … in space’ (4: 453).

Third, Kant’s proof of the persistence of corporeal substance assumes at its outset that substances cannot arise or perish, and then based on that assumption, shows that matter qua movable in space cannot arise or perish, and then concludes that matter is conserved. On its face, this sounds circular, but what Kant is doing is showing how the persistence of substance in general gives rise to a determinate physical law of conservation of matter given an empirically specific definition of corporeal substance. The point is not simply to prove persistence of (corporeal) substance, but to prove that, because substance in general persists, matter persists, and persists in a particular way, namely, as conservation of overall quantity.

Fourth, despite all Kant’s eventual work showing that we cannot prove persistence of mental substance from its concept, he starts by reiterating that the ‘object of inner sense can have a magnitude, as substance’. Where mental substance differs from material substance is that ‘its parts … are not substances’ (4: 452).

Altogether, MFNS describes what one can learn a priori from an empirically specified concept of substance (sense (3) above). The empirically specified concept of matter as movable in space proves how corporeal substance persists, namely, in the total quantity of matter being conserved rather than, say, in that any particular material objects persist for all time. For thinking substance, we cannot generate any comparable proof. The ‘parts’ of thinking substance are divided temporally, so they cannot persist through changes and are thus not themselves substances. While the category of substance must apply to the object of inner sense, we cannot specify what persists through time or in what way mental substance persists through time. From its empirical concept, we cannot give laws of persistence usable for systematic scientific study of changes in inner states. Although changes in psychological states are changes of a substance in sense (2) above, we cannot use that fact to derive a priori psychological laws.

2. The idea passage

While Kraus uses the MFNS to argue against deflationary views of material substance, the most direct support for her account of soul as mere idea occurs in the Dialectic of the first Critique, where Kant introduces three ideas by which reason thinks the systematic unity of its experience, the first of which is ‘myself, considered merely as thinking nature (soul)’ (A682/B710). Kant says about these ideas:

With respect to the soul in particular, Kant emphasizes:

Kraus brilliantly uses this passage to suggest using the idea of the soul to do for inner sense what the category of substance does for outer sense. In particular, we could organize mental states into temporal sequences through the idea of soul as substance, while we organize states of outer objects into temporal sequence through the category of substance.

While endorsing Kraus’s insight that the idea of soul allows systematic investigation of inner experience, I propose a different reading of the content of this idea. In particular, I affirm (against Kraus) that the category of substance applies literally (not as mere idea) to the substratum of our mental states. To cognize mental states as occurring in an objective rather than merely subjective succession requires that I cognize those states as states of a substance, and cognizing causal laws of inner experience requires seeing these laws as governing alterations of substance. However, the category of substance tells me only that something persists, of which these mental states/laws are states/laws. In itself, the category tells nothing about that something other than that it persists. As we saw in MFNS, we have a specific empirical concept of material substance (matter), but no corresponding empirical concept of mental substance. What’s more, from the fact that changes in mental states are alterations of substance, we don’t know whether that substance is purely mental. That, I propose, is why Kant needs the idea of the soul.

The Dialectic supports this reading. When Kant initially describes ‘the first object of such an idea’ as ‘I myself’, the reference to ‘soul’ may implicitly invoke substance, but Kant’s focus is the notion that I am ‘merely a thinking nature’ (A682/B710).

When Kant then claims, ‘I cannot … apply … categories to this object except insofar as its schema is given in sensible intuition’, he seems to rule out the application of the category of substance to ‘this object’, but the ‘object’ to which Kant refers is ‘a thinking thing … in itself’ (A682/B710). Obviously, there is no schema for cognizing a thinking thing in itself, since we lack schemata for cognizing any things in themselves. Rather than ruling out any application of categories to the soul, Kant here is making a point similar to that in the MFNS, namely, that while I might be able to apply the general and even schematized concept of substance to the self, I lack further specification of how the bare schematized notion of substance (as persisting ground of changing appearances) applies to thinking nature in particular. As he explains in the next sentence, I lack a ‘concept of experience (of what the soul actually is)’, and so I ‘cannot get very far’ in developing a ‘systematic unity of all appearances of inner sense’. Just as in the parallel discussion in MFNS, the point is not that the category of substance doesn’t apply to inner sense, but that only the barely schematized category applies, and from that category, we don’t have enough empirical content to generate psychological laws a priori, and thus don’t have enough to work with in order to systematize our psychological knowledge.

Finally, at the end of his discussion, Kant lays out four ‘principles of the systematic unity in explaining the appearances of the soul’ that the idea of soul makes possible,

Most of these ‘principles’ are not unique to investigation of mental substance. Derivation of all powers from a single power [2] is regulative for both corporeal and mental experience (A649–50/B677–8). Principles [1] and [3] might seem straightforward implications of the schematized concept of substance as described in the First and Second Analogies. However, in the context of the MFNS, it turns out that, for corporeal substance, changes occur through division and recombination of matter, so for any given set of changes, there need not be a single unchanging being that undergoes those changes (unless the world-whole is that being, in which case corporeal substance, too, is a mere idea). In his lectures on metaphysics (28: 682), and in the first Critique’s discussion of the unity of apperception, however, Kant makes clear that thinking requires an undivided thinker. To take this unity as constitutive of the thinking self as object of (possible) experience would involve a paralogism, but – as Kraus beautifully shows – Kant here uses it to construct a regulative idea for the investigation of inner states. This construction is not identical with the category of substance, even as schematized for nature in general; rather, it takes the place of the missing empirical determination of that schematized category. The idea of the soul is the mental correlate not of the category of substance as such, but of the more specific empirical concept of matter (as corporeal substance).

The fourth ‘principle’ outlined in the passage above, the most distinctive for investigation of inner appearances, is that the idea of soul ‘represent[s] all appearances in space as entirely distinct from the actions of thinking’ (A684/B712). From the appearance of inner states, we know that those states are states of inner rather than outer sense, and given the objective succession of states, we know that they are states of a substance, but we do not yet know that this substance is itself exclusively mental. In his lectures on metaphysics in 1782–3, Kant reportedly says that ‘the soul … could still be material’ (29: 908). We cannot know, either empirically or a priori, whether the substance underlying changing mental states is immaterial, but we can and should posit the idea of an exclusively mental soul.Footnote ²

Thus, when Kant goes on to say that ‘these properties could rest on entirely different grounds, with which we are not acquainted at all’ (A683/B711), I take him to include, among other possibilities, the possibility that the substance underlying the properties we ascribe to the soul could be, in some way that we cannot prove, material.Footnote ³ Unsurprisingly, then, when he catalogues the ‘advantage [that] can arise from such a psychological idea’, such as that ‘empirical laws of corporeal appearances … will not be mixed up in the explanation of what belongs merely to inner sense’, the advantages he lists do not derive from thinking of the soul as substance, but all amount to the fact that we can investigate inner appearances without getting distracted by explanations in terms of corporeal things (A683/B711). Finally, as he concludes his discussion, when he emphasizes that ‘the psychological idea can … signify nothing other than the schema of a regulative concept’, he rejects as nonsense the ascription of a constitutive concept of the soul as ‘of itself of a spiritual nature’ (A684/B712). Whether it makes sense to think of the category of substance constitutively is not at issue.

In sum, both MFNS and the Dialectic are consistent with the following view. First, in order to make sense of the objective succession of our mental states, we must take these states to be states of a substance that undergoes alterations in accordance with causal laws. This constitutive claim, based on the applicability of the Analogies to all experience, reflects ‘parity’ between inner and outer sense. Second, in order to scientifically investigate our mental lives, we ought to explain changes in mental states in terms of causal powers, and to reduce these causal powers as much as possible to a single fundamental power. Here the general regulative principle of homogeneity that governs investigation of any experience is applied specifically to inner experience. This principle, too, reflects parity between inner and outer sense. Finally, in order to maintain a focus on psychological states, laws and powers, we can and should invoke the idea of the soul as a purely mental substance. This principle is unique to the scientific investigation of inner experience. For outer experience, rather than appealing to an idea of corporeal substance, we can appeal to an empirically determined concept of matter as the movable in space. The general category of substance, like that of cause and effect, applies to inner experience, but because there is no empirically determined concept of inner substance, and in order to avoid trying to explain inner experience through appeal to corporeal substance, we posit the idea of the soul as a merely mental thing.