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 there be a second solid body, of qualities entirely different from the first—of different colour, for instance. Assume it to pass from the position &alpha;, where it gives us the aggregate of impressions A′ to the position &beta;, where it gives the aggregate of impressions B′. In general, the aggregate A will have nothing in common with the aggregate A′, nor will the aggregate B have anything in common with the aggregate B′. The transition from the aggregate A to the aggregate B, and that of the aggregate A′ to the aggregate B′, are therefore two changes which in themselves have in general nothing in common. Yet we consider both these changes as displacements; and, further, we consider them the same displacement. How can this be? It is simply because they may be both corrected by the same correlative movement of our body. "Correlative movement," therefore, constitutes the sole connection between two phenomena which otherwise we should never have dreamed of connecting.

On the other hand, our body, thanks to the number of its articulations and muscles, may have a multitude of different movements, but all are not capable of "correcting" a modification of external objects; those alone are capable of it in which our whole body, or at least all those in which the organs of our senses enter into play are displaced en bloc—i.e., without any variation of their relative positions, as in the case of a solid body.