The position taken by man outside the world to judge the world, is a factitious attitude. It is, perhaps, only a game, and one that is too easy. The division of man into two parts, thought, physical being, one considering the other and pretending to contain it, is only a philosophic amusement which becomes impossible the moment we stop to consider. There is, in fact, a physic of thought. We know that it is a product, measurable, ponderable. Unformulated externally, it nevertheless manifests its physical existence by the weight which it imposes upon the nervous system. It needs speech, writing, or some sort of sign, in order to manifest itself externally. Telepathy, thought, penetration, presentiment--if there be any facts in this category which have really been verified--would in such a case be so many proofs of the materiality of thought. But it is useless to multiply arguments in favour of a fact which is no longer contested save by theology. This fact of materiality gives thought a secondary place. It is produced. It might not be. It is not primordial. It is a result, a consequence--doubtless a property of the nervous system, or even of living matter. It is then through a singular abuse, that we have become accustomed to consider it isolated from the ensemble of its producing causes. But, if thought be a product, it is, none the less, productive in its turn. It does not create the world, it judges it. It does not destroy it, it modifies and reduces it to its measure. To know is to frame a judgment; but every judgment is arbitrary, since it is an accommodation, an average, and since two different physiologies give different averages, just as they give different extremes. The path, once again, after many windings, brings us back to idealism. Idealism is definitely founded on the very materiality of thought, considered as a physiological product. The conception of an external world exactly knowable is compatible only with belief in the reason, that is to say, in the soul, that is to say, furthermore, in the existence of an unchangeable, incorruptible, immortal principle, whose judgments are infallible. If, on the contrary, knowledge of the world be the work of a humble physiological product, thought--a product differing in quality, in modality, from man to man, species to species--the world may perhaps be considered as unknowable, since each brain or each nervous system derived from its vision and from its contact a different image, or one which, if it was at first the same for all, is profoundly modified in its final representation by the intervention of the individual judgment. If the same object produces the same image on the retina of an ox or the retina of a man, it will not, doubtless, be concluded, therefore, that this image is known and judged identically by the ox and the man.