My personal opinion is that sensation is of a mixed nature. It is psychical in so far as it implies an act of consciousness, and physical otherwise. The impression on which the act of cognition operates, that impression which is directly produced by the excitant of the nervous system, seems to me, without any doubt, to be of an entirely physical nature. This opinion, which I make mine own, has only been upheld by very few philosophers--Thomas Reid perhaps, and William Hamilton for certain; but neither has perceived its deep-lying consequences. What are the arguments on which I rely? They are of different orders, and are arguments of fact and arguments of logic. I shall first appeal to the natural conviction of those who have never ventured into metaphysics. So long as no endeavour has been made to demonstrate the contrary to them, they believe, with a natural and naive belief, that matter is that which is seen, touched and felt, and that, consequently, matter and our senses are confounded. They would be greatly astonished to be informed that when we appear to perceive the outer world, we simply perceive our ideas; that when we take the train for Lyons we enter into one state of consciousness in order to attain another state of consciousness. Now, the adherents of this natural and naive opinion have, as they say in the law, the right of possession (_possession d'etat_); they are not plaintiffs but defendants; it is not for them to prove they are in the right, it has to be proved against them that they are in the wrong. Until this proof is forthcoming they have a presumption in their favour.