Man, like all the rest of nature, is an entirely material being. The mind, the facility of thinking, of receiving and reflecting upon different external and internal sensations, of remembering them when they have passed and reproducing them by the imagination, of comparing and distinguishing them, of abstracting determinations common to them and thus creating general concepts, and finally of forming ideas by grouping and combining concepts according to different methods--intelligence, in a word, sole creator of our whole ideal world, is a property of the animal body and especially of the quite material organism of the brain. We know this certainly, by the experience of all, which no fact has ever contradicted and which any man can verify at any moment of his life. In all animals, without excepting the wholly inferior species, we find a certain degree of intelligence, and we see that, in the series of species, animal intelligence develops in proportion as the organization of a species approaches that of man, but that in man alone it attains to that power of abstraction which properly constitutes thought. Universal experience,[8] which is the sole origin, the source of all our knowledge, shows us, therefore, that all intelligence is always attached to some animal body, and that the intensity, the power, of this animal function depends upon the relative perfection of the organism. The latter of these results of universal experience is not applicable only to the different animal species; we establish it likewise in men, whose intellectual and moral power depends so clearly upon the greater or less perfection of their organism as a race, as a nation, as a class, and as individuals, that it is not necessary to insist upon this point.[9] On the other hand, it is certain that no man has ever seen or can see pure mind, detached from all material form, existing separately from any animal body whatsoever. But if no person has seen it, how is it that men have come to believe in its existence? The fact of this belief is certain, and if not universal, as all the idealists pretend, at least very general, and as such it is entirely worthy of our closest attention, for a general belief, however foolish it may be, exercises too potent a sway over the destiny of men to warrant us in ignoring it or putting it aside.