Not only could this notion not be derived from a single sensation or even _from a series of sensations_, but what is more, an _immobile_ being could never have acquired it, since, not being able to _correct_ by his movements the effects of the changes of position of exterior objects, he would have had no reason whatever to distinguish them from changes of state. Just as little could he have acquired it if his motions had not been voluntary or were unaccompanied by any sensations. CONDITIONS OF COMPENSATION.--How is a like compensation possible, of such sort that two changes, otherwise independent of each other, reciprocally correct each other? A mind already familiar with geometry would reason as follows: Evidently, if there is to be compensation, the various parts of the external object, on the one hand, and the various sense organs, on the other hand, must be in the same _relative_ position after the double change. And, for that to be the case, the various parts of the external object must likewise have retained in reference to each other the same relative position, and the same must be true of the various parts of our body in regard to each other. In other words, the external object, in the first change, must be displaced as is a rigid solid, and so must it be with the whole of our body in the second change which corrects the first. Under these conditions, compensation may take place. But we who as yet know nothing of geometry, since for us the notion of space is not yet formed, we can not reason thus, we can not foresee _a priori_ whether compensation is possible. But experience teaches us that it sometimes happens, and it is from this experimental fact that we start to distinguish changes of state from changes of position. SOLID BODIES AND GEOMETRY.--Among surrounding objects there are some which frequently undergo displacements susceptible of being thus corrected by a correlative movement of our own body; these are the _solid bodies_. The other objects, whose form is variable, only exceptionally undergo like displacements (change of position without change of form). When a body changes its place _and its shape_, we can no longer, by appropriate movements, bring back our sense-organs into the same _relative_ situation with regard to this body; consequently we can no longer reestablish the primitive totality of impressions. It is only later, and as a consequence of new experiences, that we learn how to decompose the bodies of variable form into smaller elements, such that each is displaced almost in accordance with the same laws as solid bodies. Thus we distinguish 'deformations' from other changes of state; in these deformations, each element undergoes a mere change of position, which can be corrected, but the modification undergone by the aggregate is more profound and is no longer susceptible of correction by a correlative movement.