According to Kant’s notion of categorical judgment, subject gives us the notion of a substance, which is the absolute subject of predication and not a property/predicate of anything else. Descartes argues that because one thinks, one is a thinking substance. His logic is that since “I” is the absolute subject of all judgments, the predicate is true (i.e. having thoughts). “I” is not the property of anything else, so it is a substance (i.e. the soul). Kant believes the premise is non-controversial and leads to the conclusion that one is a thinking subject. To represent empirical objects as concepts, one must have thoughts to give form to and unite an unorganized manifold of sensations through judgments with logical forms. To have a thought necessitates unifying representations, which “requires unity of consciousness in the synthesis of them” (B137 Kant). If one is a thinking subject, one has the thoughts (249 Kant).
However, Kant thinks it is illegitimate that Descartes argues that being a subject of thought indicates one is a thinking substance. “I think” is no basis for the existence of a non-empirical substance, which is unknowable. One does not experience themselves as a soul but only as a thinking subject. Thus, Kant treats the doctrine of the soul as purely rational (i.e. non-empirical).
The ground “I think” is a perception of one’s “inner experience.” Although to have the inner experience of thinking requires the unity of consciousness/transcendental subject, Kant contends that humans can only determine any knowledge about any objects only by using the categories with empirical grounds, so any of our knowledge about objects is limited to empirical objects. Descartes may wonder about to which object the attribute of thinking belongs, but asserting that this object is the thinking substance, which is non-empirical and completely inaccessible to human perception, is illegitimate.
Since philosophers derive the notion of substance from categories, why can’t Descartes argue legitimately for the existence of a thinking substance based on a thinking subject? Categories are a priori concepts of understanding that make judgments and organize intuitions that represent individual objects, such as time and space. The notion of substance can be used to talk about either empirical and non-empirical objects. Kant believes humans can acquire legitimate knowledge through judgments about substances in the empirical world because the categories are applied to empirical objects. However, combining empirical intuitions with the categories only allows us to make empirical judgments to learn about empirical objects. If a judgment contains no intuition of an object, the judgment cannot be attributed to some object, so there is no knowledge to discover about any object, including the existence of an object. To yield true knowledge about an object, only the empirical usage scenario of the notion of substance is legitimate.
Kant believes any empirical ground “mixed among the grounds of cognition” of the rational doctrine of the soul makes the doctrine of the soul no longer purely rational (412 Kant). So to use “I think” to prove the existence of a non-empirical object necessitates using the categories in a purely rational manner based on rational grounds. This is impossible because humans can only find empirical grounds to prove the existence of empirical objects. Descartes has mistaken the unity of consciousness for an intuition of a thinking substance, which is non-empirical and provides no intuition perceivable by humans. Thus, his argument says nothing about the soul, including its existence. The unity of consciousness does not represent any object but is merely a vehicle for representing the manifold as a thought. Thus, “I think” does not suggest any intuition that originates from any substance. Any attempt to apply our a priori notion of substance based on the unity of consciousness, which does not serve as an experience of a substance, to investigate about a thinking substance, which is beyond our experience, may give us a concept of a thinking substance. But such a concept by itself provides no knowledge about any object, including its existence, because this concept is conceived before we have perceived a relevant object in reality through an intuition.
This paper will argue that Descartes’ argument can be used to prove that the soul is an empirical substance. Kant has mentioned that the postulation of the existence of magnetic matter before perceiving magnetic directly by perceiving attracted iron filings is legitimate because it is based on empirical objects we have intuitions of and empirical laws. The magnetic matter here is inferred from and connected to our experience of empirical objects. This is why if Kant is alive, he will accept that electromagnetic waves, an entity inferred from empirical laws and intuitions, are a substance even if we do not have any intuition of them.
Similarly, the concept of “I” is acquired through experience. If a thinking subject enters a deep meditative state, the concept of “I” disappears, and there are no thoughts. The only sensation left is pure consciousness (i.e. merely a sensation of being aware). Although the subject is not thinking, the subject feels conscious but discontinues experiencing itself as an individual or as anything. The concept “I” is shu acquired through experience, so Descartes’ argument has an intuition for inferring a thinking subject. The thinking subject can enter a mental state where there is no intuition of external objects, concepts and sensations and where the concept “I” disappears but the subject still experiences pure consciousness. This can occur because even though humans have an a priori concept of space that allows one to feel separate from other things, without external objects, there is nothing to feel separate from. Without an empirical external object (e.g. an arm) to allow the thinking subject to experience sensations from what’s outside oneself, the thinking subject cannot have a concept of “I.” Thus, the unity of consciousness is empirical because humans take an intuition of at least one external object for the concept “I” to emerge. The empirical judgment that “I am a thinking substance” thus is based on an intuition of empirical objects that allow the concept “I” to emerge. Hence, the soul is similar in nature to electromagnetic waves. One cannot experience it directly, but one can infer its existence based on our empirical intuition and understanding of an external object and exercising the categories. Our intuition of external objects in general allows us to infer that one is an empirical thinking substance.
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