As we saw in Chapter 3, what Kant referred to as his “Copernican revolution” in philosophy was motivated by the matching problem. According to Kant, if our knowledge-acquiring faculties are capable of yielding knowledge of the world, as they certainly seem to be, then this must be because they must, in some way, match up to the world. There must be some sort of fit or match between the nature of our knowledge-acquiring faculties and the nature of the world. The question of how knowledge is possible, then, translates into a question about how this matching can occur. Kant's answer to this was his transcendental idealism. Our knowledge-acquiring faculties – the faculties of sensibility and understanding – match up, or mesh, with the world because the world, the phenomenal world of our everyday experience, is constructed by these faculties.

The Kantian, idealist, solution to the matching problem is, then, based on the idea that phenomenal or empirical items are not individuated independently of the activities of the mind. And they are not individuated independently of the activities of the mind because they are, in part, products of those activities. In the work of Sartre, examined in Chapter 4, we see the outlines of an alternative solution to the matching problem. Again, the solution is based on the idea that environmental items are not individuated independently of mental items. But, according to Sartre, this lack of individuative independence is for precisely the converse reason. Consciousness, for Sartre, is nothing more than directedness towards the world.