By the very fact that we conceive a thing, which is like ourselves, and which we have not regarded with any emotion, to be affected with any emotion, we are ourselves affected with a like emotion (affectus).
Proof .-- The images of things are modifications of the human body, whereof the ideas represent external bodies as present to us (II.

xvii.); in other words (II.

x.), whereof the ideas involve the nature of our body, and, at the same time, the nature of the external bodies as present.

If, therefore, the nature of the external body be similar to the nature of our body, then the idea which we form of the external body will involve a modification of our own body similar to the modification of the external body.

Consequently, if we conceive anyone similar to ourselves as affected by any emotion, this conception will express a modification of our body similar to that emotion.
Thus, from the fact of conceiving a thing like ourselves to be affected with any emotion, we are ourselves affected with a like emotion.