At least for Descartes, the quest for certitude requires initially dismissing the entire order of sense and bodily experience, even my experience of myself as a body. The putting into question of the entire physical order allows the pristine intelligibility of the order of mind to come to the fore. We know ourselves better than other things and we know out intellect better than our body. The intellectual self is known immediately and transparently. Following Aristotle, and in contrast to Descartes, Aquinas urges a methodological retreat in our pursuit of self-knowledge. There is no possibility of gaining immediate, introspective access to the intellect or the soul. The route to self-knowledge is indirect, oblique. To understand the essence of any species, we must begin with the objects naturally pursued by members of the species in question, then move back from these to examine the activities, powers, and, finally, the essence. The indirect route to self-knowledge follows from the fact that the intellect is a potency made actual only by knowing things. But a power is knowable in so far as it is in act. Thus, there is no possibility of knowing the intellect until it has been actualized by knowing something other than itself.The indirect and mediated path to knowledge of the human soul does not diminish the importance of that knowledge. Indeed, the general investigation of soul culminates with an analysis of what is proper to human souls. Thus we find Aquinas explicating in great detail Aristotle's comparison of sensation and understanding and his argument that intellect so differs from sense that it must be an immaterial power, whose operation transcends every bodily organ. Like sense, the intellect is said to be passive with respect to sensible objects. It is a potency actualized by receiving the forms of things. But there are different sense of passivity and clarification of them is crucial to a comparison of sense and intellect. Aquinas writes:
To be passive may be taken in three ways. First, in its most strict sense, when from a thing is taken something which belongs to it by virtue either of its nature, or of its proper inclination, as when ...a man becomes ill. Secondly, less strictly, a thing is said to be passive when something either suitable or unsuitable is taken away from it. And in this way not only he who is ill is said to be passive, but also he who is healed.... Thirdly, in a wide sense a thing is said to be passive, from the very fact that what is in potency to something receives that to which it was in potency without being deprived of anything. And accordingly whatever passes from potency to act may be said to be passive, even when it is perfected. And thus with us to understand is to be passive (ST, I, 79, 2).
But how can one demonstrate the fact that one thinks?
They cannot. It is impossible to "prove" that one exists, because it is impossible to "prove" that one thinks. We directly experience our own existence -- but it is impossible to demonstrate that existence scientifically. We all take our own existence on faith.
Therefore, even rationalism is based on unprovable faith.
B-chan