The ontological analysis of the body is one of the important theses of Cartesianism. This analysis is an essential analysis, it leads to bringing extension to light as the essence of the body. There is no need to distinguish several forms of extension, it is the same extension which we conceive or which we imagine, the difference lies in the act of the mind which apprehends it and not in the nature of extension as such. Extension is the milieu wherein movements take place, movements which are purely mechanical and in every case amount to a displacement of the different parts of extension, one pushing the other; this latter operation takes place instantaneously. The ontological determination of the essence of the body as extension has an absolutely general meaning in Cartesianism: That the body must be understood essentially as extension does not hold only for the inert body of physical nature, this affirmation also applies to the living body and the human body. The result is, in one case, the famous theory of animal-machines and, when it comes to the human body, the conception of the latter as an assemblage in extension of extended parts bound to one another according to a mechanical relationship. Actually, there is no difference between the human body and the body of the animal, any more than there is between the latter and any physical body whatever. [190]