cally, that is by extension, form, and motion, just as Democritus had done before; but he did not go farther, finding in extension the very essence of corporeal substance. Leibniz's genius showed itself when he pointed out that extension does not suffice to explain phenomena and that it has need itself of an explanation. Brought up in the scholastic and peripatetic philosophy, he was naturally predisposed to accord more of reality to the corporeal substance, and his own reflections soon carried him much farther along the same line.
It is also worth noticing, as Guhrauer has said in his Life of Leibniz, that it was a theological problem which put Leibniz upon the track of reforming the conception of substance. The question was rife as to the real presence in transubstantiation. This problem seemed inexplicable upon the Cartesian hypothesis, for if the essence of a body is its extension, it is a contradiction that the same body can be found in several places at the same time. Leibniz, writing to Arnauld in 1671, says he thinks he has found the solution to this great problem, since he has discovered "that the essence of a body does not consist in extension, that the corporeal substance, even taken by itself, is not extension and is not subject to the conditions of extension. This would have been evident if the real character of substance had been discovered sooner."
Leaving aside this point, however, the following are the different considerations which led Leibniz to admit non-mechanical principles as above corporeal mechanicalism, and to reduce the idea of the body to the idea of active indivisible substances, entelechies or monads, having innate within themselves the reason for all their determinations.
1. The first and principal reason which Leibniz brings up against Descartes is that, "If all that there is in bodies is extension and the position of the parts, then when two bodies come into contact and move on together after the contact, that one which was in motion will carry along the body at rest without losing any of its velocity, and the difference in the sizes of the bodies will effect no change," which is contrary to experience. A body in motion which comes in contact with one at rest loses some of its velocity and its direction is modified, which would not happen if the body were purely passive. "Higher conceptions must therefore be added to extension, namely, the conceptions of substance, action and force; these
