And yet we regard these two changes both as displacements and, furthermore, we consider them as the same displacement. How can that be?
It is simply because they can both be corrected by the same correlative movement of our body.
'Correlative movement' therefore constitutes the sole connection between two phenomena which otherwise we never should have dreamt of likening.
On the other hand, our body, thanks to the number of its articulations and muscles, may make a multitude of different movements; but all are not capable of 'correcting' a modification of external objects; only those will be capable of it in which our whole body, or at least all those of our sense-organs which come into play, are displaced as a whole, that is, without their relative positions varying, or in the fashion of a solid body.
To summarize:
1º We are led at first to distinguish two categories of phenomena:
Some, involuntary, unaccompanied by muscular sensations, are attributed by us to external objects; these are external changes;
Others, opposite in character and attributed by us to the movements of our own body, are internal changes;
2º We notice that certain changes of each of these categories may be corrected by a correlative change of the other category;
3º We distinguish among external changes those which have thus a correlative in the other category; these we call displacements; and just so among the internal changes, we distinguish those which have a correlative in the first category.