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. [Sidenote: To understand the intensity, duration and voluntary determination of psychic states, we must eliminate the idea of space.] Now just as, in order to ascertain the real relations of physical phenomena to one another, we abstract whatever obviously clashes with them in our way of perceiving and thinking, so, in order to view the self in its original purity, psychology ought to eliminate or correct certain forms which bear the obvious mark of the external world. What are these forms? When isolated from one another and regarded as so many distinct units, psychic states seem to be more or less _intense._ Next, looked at in their multiplicity, they unfold in time and constitute _duration._ Finally, in their relations to one another, and in so far as a certain unity is preserved throughout their multiplicity, they seem to _determine_ one another. Intensity, duration, voluntary determination, these are the three ideas which had to be clarified by ridding them of all that they owe to the intrusion of the sensible world and, in a word, to the obsession of the idea of space. [Sidenote: Intensity is quality and not quantity or magnitude.]