Now, in the course of this inventory, we perceive that we have continually to do with two orders of elements, which are united in reality, but which our thought may consider as isolated. One of these elements is represented by those states which we designate by the name of sensations, images, emotions, &c.; the other element is the consciousness of these sensations, the cognition of these images, the fact of experiencing these emotions. It is, in other words, a special activity of which these states are the object and, as it were, the point of application--an activity which consists in perceiving, judging, comparing, understanding, and willing. To make our inventory orderly, let us deal with these two elements separately and begin with the first. We will first examine sensation: let us put aside that which is the fact of feeling, and retain that which is felt. Thus defined and slightly condensed, what is sensation? Until now we have employed the word in the very vague sense of a _tertium quid_ interposed between the object and ourselves. Now we have to be more precise, and to inquire whether sensation is a physical or a mental thing. I need not tell you that on this point every possible opinion has been held. My own opinion is that sensation should be considered as a physical phenomenon; sensation, be it understood, in the sense of impression felt, and not in that of capacity to feel. Here are the arguments I invoke for the support of my thesis: in the first place, popular opinion, which identifies matter with what we see, and with what we touch--that is to say, with sensation. This popular opinion represents a primitive attitude, a family possession which we have the right to retain, so long as it is not proved to us to be false: next, this remark, that by its mode of apparition at once unexpected, the revealer of new cognitions, and independent of our will, as well as by its content, sensation sums up for us all we understand by matter, physical state, outer world. Colour, form, extent, position in space, are known to us as sensations only. Sensation is not a means of knowing these properties of matter, it is these properties themselves. What objections can be raised against my conclusion? One has evidently the right to apply the term psychological to the whole sensation, taken _en bloc_, and comprising in itself both impression and consciousness. The result of this terminology will be that, as we know nothing except sensations, the physical will remain unknowable, and the distinction between the physical and the mental will vanish. But it will eventually be re-established under other names by utilising the distinction I have made between objects of cognition and acts of cognition;--a distinction which is not verbal, and results from observation.