One would be greatly deceived if he reduced this difference to the ordinary opposition between the physical and the spiritual, between external and internal phenomena. Sensation, the foundation and the starting point of the objective order of knowledge, is just as internal as volition. On the other hand, man is a part of what we call Nature; and, as such, he is the theatre of a crowd of internal and external phenomena which, so far as that is possible, should be observed, described, explained, by the principle of causality, like all the other phenomena of the physical order. For example, the mechanism of memory and that of logic, the correlation between mental activities and the physiological modifications of the cerebro-spinal system, the laws of association of ideas, the stable forms of the human understanding, all that psychology that is now called "scientific psychology," rightfully enters into the domain of the sciences of Nature. It is a province that may be explored like all the others. The psychological observations made in it are objective not less than those of physiology, for the reason that the phenomena that are observed, while occurring in the ego, are nevertheless produced in it without the voluntary intervention of the ego, and even without its express consent. Moreover, they do not imply or provoke on the part of the ego any moral judgment properly so called. On the other hand, take the sciences of Nature which deal with the objects most widely removed from man, with astronomy or geology, _e.g._; no longer consider the bare external results; consider rather that spiritual force which we call thought, and which has the virtue of producing these sciences; what are they but the external revelation of the creative and organising energy of the thinking subject, the revelation of spirit to spirit? The work, seen from this subjective side, serves simply to set forth the worth of the worker. You speak then of the ordinary savant or of the intellectual genius, of the good or bad scientific workman. The philosophy of science becomes a necessarily subjective discipline. "Science," in fact, is simply an abstraction. In the reality there are only minds more or less ignorant, conscious, at each step, of their strength and of their impotence, of their defeats and victories,--minds condemned to a perpetual effort to struggle out of the night from which they slowly mount. When you think of this most disinterested side of the scientific life you ask yourself what is the basis, in the last resort, of this confidence of mind in itself--the foundation of all the rest. You see clearly that this activity of pure intellect demands, like all other human activity, attention, forgetfulness of self, a heroism, in short, going to the point of contempt of common enjoyments, and of the sacrifice of life itself. You have then left the domain of the sciences of Nature and have entered the realms of spirit, and there