The mental knot Kant associates with psychological ideas is that of substance, and particularly that of a thinking substance. Talk of substances was a major preoccupation of the rationalist metaphysics of the 17th and 18th centuries, and Descartes was one of the major philosophers to discuss substances. Descartes is famous for the statement "I think therefore I am": I cannot doubt that I exist, since the act of doubting is an act of thought and I couldn't think unless I existed. I exist: but what can I know about this "I" that I am? While I know that I think, I can doubt that I have a body (I could be a butterfly dreaming I have this body), so I conclude that I am a thinking (as opposed to bodily) substance. I may think I know, or I may guess, any number of things about my body, but while these thoughts or guesses may be mistaken, I cannot doubt that I am thinking or guessing. From this line of reasoning, I conclude that my mind is better known to me than my body.
And so on. In the Meditations, Descartes questions the reliability of the senses, and then attempts to see how much he can know about himself and the world around him using only his intellect.
According to Kant, all I can know about this "I" is that I am. What I sense and think are representations, and these representations have to take place within a subject. For things to be seen and heard there has to be a consciousness that is doing the seeing and hearing. This "I" essentially represents that logical necessity: there must be something doing the seeing and hearing, and I call that something "I."
This "I" is not something I encounter in experience; it is the basis for my experience. As a result, we cannot apply to it the categories we apply to experience. Descartes tries to do essentially that, applying the concept of substance and other concepts of the pure understanding to it. Kant suggests on the contrary that we should think of this "I" the way we think of things in themselves: we can infer that it is, but we cannot infer anything about it. Pure reason, engaging in metaphysics, cannot tell us anything substantial about the way things are.
