Descartes expressed his inability to doubt himself at the end of a process in which he had doubted everything sensible by saying, “I think therefore I am.” He then questioned what he is and deduced himself to be something that thinks, namely a soul, mind or intellect. However, Kant reconstructed Descartes’ reasoning, which deduced himself to be a spiritual substance from the statement “I think” as follows: Things that are used as subjects in judgment are substances, I am also a subject in a judgment, therefore I am a substance. Kant characterized this reasoning as a paralogism (i.e., logically false inference) and used a similar version in the second edition of his work. Accordingly, the version in the first edition is called version A, and the version in the second edition is called version B. By defining substance as a category in his version A, Kant accepted that inference validly proves the conclusion “I am a substance” as the logical subject of judgment but he stipulated that it should not be deduced from this inference that “I” (i.e., the soul) am a being that remains the same under all changes, or that I am even immortal. For Kant, the conclusion that the “soul is a substance” is accordingly valid unless it is understood as an immortal being. For this reason, some commentators have argued that Kant found this inference valid, while others focus on presenting it as a logically wrong inference. Those who argue the inference to be valid here accept Kant’s definition of substance as a category. Contrary to them, those who focus on logical fallacy think that acknowledging “I” to be a substance will also lead to the conclusion of “I am an immortal being.” This situation does not come from the definition of substance made by Kant; on the contrary, it fits the Aristotelian definition of substance, because substance, as defined by Aristotle, refers to things that exist by themselves. Reading the inference according to this definition of substance leads to the conclusion that I am a substance and therefore immortal. However, Kant argued the concept of substance has no intuitive content for drawing this conclusion. In other words, no intuition is found for the soul as a substance. If one is to understand this inference according to the Aristotelian definition of substance, then this is the reason why Kant presented this inference as a logically false one. In version B of the inference, Kant replaced the expression “I think” with the expression “That which thinks,” and claimed the inference to contain an error in the form of reasoning: namely the error of using the middle term to have a double meaning in the premises. However, Kant’s inference does not show the middle term to be used with a double meaning. According to him, this inference deduces that the thinking thing is also a substance from the substantiality of the intuition objects which are the subjects because the thinking thing is the subject. Accordingly, Kant seems to invalidate the inference of the substantiality of the soul because it has no intuition. However, this is not a logical fallacy; therefore, it is more correct for Kant’s objection to be epistemological rather than logical. This is because Kant accepts the major premise as knowledge due to it having intuition while accepting the minor premise as only thought due to it possessing no intuition according to his theory of knowledge. Accordingly, the inference is not invalid, but its conclusion can be considered devoid of knowledge value according to Kant’s epistemology. In this regard, Kant’s acceptance of version A as valid according to his own definition while invalidating version B shows that he had used the concept of substance according to the Aristotelian definition here.

But is the inference that Kant himself constructed and considered logically wrong to be Descartes’ argument? Kant claims “I” is a substance to be deduced in the conclusion based on the substantiveness of the intuitional objects used as subjects in the judgment and the pronoun “I” being used as the subject in the judgment. On the other hand, Descartes had concluded “I think, therefore I am” due to his inability to doubt himself while doubting everything sensible. When he later questioned what he is, he noticed that only thinking is inseparable from himself (i.e., undoubtable). Accordingly, Descartes deduced himself to be a thinking thing, namely a soul, intellect, or mind. Here, Descartes’ statement that he is a thinking thing, namely spirit, intellect, or mind, aims to express his own existence as something insensible, because even when he doubts everything sensible, his inability to doubt himself reveals his own existence to be something insensible. The truth is that what is doubtful cannot be the same as what is indubitable. If that were the case, Descartes would also need to doubt himself when he doubts everything sensible. Accordingly, Descartes’ argument can be shown as follows: Sensible things (intuitional objects) are doubtful; my existence is not doubtful, so my existence is not sensible (intuitional object); therefore, I am a soul, mind, or intellect. As can be seen in this way, Descartes’ conclusion “I am a soul,” or in other words a substance, does not rely on the substantiality of intuitive things as Kant’s inference does. On the contrary, Descartes expresses it as a spiritual substance in order to distinguish it from intuitional objects. Therefore, Kant’s paralogism of substantiality is not Descartes’ argument.

However, Kant’s critique expresses the result that “I am a substance” (i.e., the soul) is not knowable. Kant does not want to regard the substantiality of the soul as knowledge due to the absence of the intuitive content that he requires in order to know anything. This is because, for Kant, “I think” is consciousness and not intuition. But why does Kant want to see this result as unknowable? This is apparently because Kant mainly wanted to prevent materialism from attributing “what thinks” to matter. However, this agnostic attitude does not seem strong enough to prevent materialism at all, because this agnosticism means that the consciousness of “I think” can also be produced by a nonthinking substance or may arise all by itself out of nothing. In this case, how a nonthinking substance causes the consciousness of “I think” or how the consciousness of “I think” arises by itself out of nothing is unexplainable because absence is not the cause of anything. Therefore, consequences for the human mind demand ontologically equivalent causes. Accordingly, the consciousness of “I think” is expected to be produced by a thinking substance. Likewise, Kant accepts the equality of quantities in the changes of matter or the equality of action and reaction in motion, as a synthetic a priori knowledge that is not empirical and necessary. In this context and also for Kant, one can say that the cause of thought is a thinking substance (i.e., the soul) can be known as a necessary knowledge that is not based on experience.