According to Kant, knowledge includes matter and form. Its matter comes form the sense, and the forms are drived from perception and understanding: In perception we have two intuitional categoties that called space and time, and in the stage of understanding 12 other categories. What we know in this process is phenomenon not nomenon (the thing in itself). However, our reason, by its nature, transcends from the realm of phenomenon, and constructs ideas, which are, in fact, illusions. Such an idea is the idea of the soul. Reason is intended by means of four paralogical demonstrations, proves four subjects about the soul: Substantiality, simplicity, identity, and its distinction from matter (the soul has certain existence but the matter has doubtful existence).
"I think" or "I", which according to Kant is an apriori and formal condition of knowledge, is the origin of all these paralogisms, and assums an objective thing or a subject of knowledge. Moreover, in the Fourth paralogism, Kant disagrees with idealism in its current meaning, by proposing the theory of transcendental idealism, and considers the ideas of the mind as external in a particular sense.