To sum up the foregoing discussion, we shall put aside for the present Kant's terminology and also his doctrine, to which we shall return later, and we shall take the point of view of common sense. Modern psychology seems to us particularly concerned to prove that we perceive things through the medium of certain forms, borrowed from our own constitution. This tendency has become more and more marked since Kant: while the German philosopher drew a sharp line of separation between time and space, the extensive and the intensive, and, as we should say to-day, consciousness and external perception, the empirical school, carrying analysis still further, tries to reconstruct the extensive out of the intensive, space out of duration, and externality out of inner states. Physics, moreover, comes in to complete the work of psychology in this respect: it shows that, if we wish to forecast phenomena, we must make a clean sweep of the impression which they produce on consciousness and treat sensations as signs of reality, not as reality itself.
It seemed to us that there was good reason to set ourselves the opposite problem and to ask whether the most obvious states of the ego itself, which we believe that we grasp directly, are not mostly perceived through the medium of certain forms borrowed from the external world, which thus gives us back what we have lent it. A priori it seems fairly probable that this is what happens. For, assuming that the forms alluded to, into which we fit matter, come entirely from the mind, it seems difficult to apply them constantly to objects without the latter soon leaving a mark on them: by then using these forms to gain a knowledge of our own person we run the risk of mistaking for the colouring of the self the reflection of the frame in which we place it, i.e. the external world. But one can go further still and assert that forms applicable to things cannot be entirely our own work, that they must result from a compromise between matter and mind, that if we give much to matter we probably receive something from it, and that thus, when we try to grasp ourselves after an excursion into the external world, we no longer have our hands free.