Kant's Conception of the Subject

In a few passages of the Critique of Pure Reason,¹ Kant seems to treat the terms "soul" and "subject" as identical; for example, when he speaks in a general way of the faculties or capacities of the soul (see, e.g., Kant 1998, A94/B127; A124; A141/B180; A406/B432). The majority of the time, however, Kant claims that the soul is an object of inner sense (see, e.g., Kant 1998, A22/B37; A361; A381; A385; A846/B874); that is, that the soul is identical to the thinking I precisely as it appears to itself under the form of time. In this case, Kant is referring to the phenomenal or psychological I, and he cleanly distinguishes this from the I of transcendental apperception, that is, from the transcendental subject of thoughts = x, which cannot be the object of self-knowledge. In the chapter on the "Paralogisms of Pure Reason," Kant critiques the concept of soul from the rational psychology of his time, which conceives of the thinking I as a simple substance, numerically identical throughout time and whose existence, unlike that of the body, is indubitable. This rational concept of the soul or subject, according to Kant, belongs to the field of noumena and completely exceeds the limits of our knowledge (Kant 1998, B430–32).

Thus, the concept of the subject in Kant always refers to a self-consciousness that in principle should be able to express itself through the term "I." In this connection, three types of self-consciousness can be distinguished: transcendental self-consciousness (or apperception), empirical self-consciousness, and the supposed self-consciousness of the I as it is in itself, that is, the sort of self-consciousness that would imply the possibility that the subject knows itself precisely as it is in itself. Consequently, there would also be three modes of conceiving such a thinking I: as the transcendental I, as the phenomenal I, and as the noumenal I.

Now, Kant holds that none of these types of self-consciousness give rise to an authentic theoretical sort of knowledge. In fact, transcendental self-consciousness is just the form or vehicle of all objective representation, a transcendental condition of knowledge that cannot itself be an object of knowledge. For its part, neither can the phenomenal I be an object of knowledge in the strict sense, because there is nothing stable or permanent in it, as Kant so often points out. In effect, the I that tries to grasp itself in inner sense is something that, so to speak, constantly escapes our observation in a sort of perpetual circle (Kant 1998, A346/B404), such that this I is never the true substrate of thinking activity, or at most is just a part of what we ourselves are.

For the thinking I to be an object of self-knowledge, it should be and exhibit itself as a substance, but Kant shows that neither the transcendental I nor the phenomenal one can be considered substances, for different reasons. Having said that, precisely in the chapter on the paralogisms, dedicated to critiquing the metaphysical conception of the soul and in particular the Cartesian concept of the I as thinking substance, Kant also claims that, in spite of all this, the conclusive propositions of said paralogisms may well continue to hold if they are understood in the right way. Put otherwise, there at least would be a sense in which we indeed could refer to ourselves as if we were real, simple, and personal substances, that is, souls. How should we flesh this claim out?

At first glance, it makes sense to think that Kant here is referring only to the idea of the soul that will be treated further on in the chapter on the "Canon of Pure Reason" and later in the Critique of Practical Reason, in the context of practical philosophy. In that case, what would be at issue is an idea that only has authentic validity in the practical use of reason, under the form of a moral belief or of a postulate regarding the immortality...