Now, in the continuity of sensible qualities we mark off the boundaries of bodies. Each of these bodies really changes at every moment. In the first place, it resolves itself into a group of qualities, and every quality, as we said, consists of a succession of elementary movements. But, even if we regard the quality as a stable state, the body is still unstable in that it changes qualities without ceasing. The body pre-eminently--that which we are most justified in isolating within the continuity of matter, because it constitutes a relatively closed system--is the living body; it is, moreover, for it that we cut out the others within the whole. Now, life is an evolution. We concentrate a period of this evolution in a stable view which we call a form, and, when the change has become considerable enough to overcome the fortunate inertia of our perception, we say that the body has changed its form. But in reality the body is changing form at every moment; or rather, there is no form, since form is immobile and the reality is movement. What is real is the continual _change of_ form: _form is only a snapshot view of a transition_. Therefore, here again, our perception manages to solidify into discontinuous images the fluid continuity of the real. When the successive images do not differ from each other too much, we consider them all as the waxing and waning of a single _mean_ image, or as the deformation of this image in different directions. And to this mean we really allude when we speak of the _essence_ of a thing, or of the thing itself.