P41: If someone imagines that someone loves him, and does not believe he has given any cause for this, he will love {that person} in return.

Dem.: This Proposition is demonstrated in the same way as the preceding one. See also its scholium.

Schol.: But if he believes that he has given just cause for this Love, he will exult at being esteemed (by P30 and P30 S). This, indeed, happens rather frequently (by P25) and is the opposite of what we said happens when someone imagines that someone hates him (see P40 S).

Next, this reciprocal Love, and consequent (by P39) striving to benefit one who loves us, and strives (by the same P39) to benefit us, is called Thankfulness, or Gratitude.

And so it is evident that men are far more ready for Vengeance than for returning benefits.

Cor.: He who imagines he is loved by one he hates will be torn by Hate and Love together. This is demonstrated in the same way as P40 C1.

Schol.: But if the Hate has prevailed, he will strive to do evil to the one who loves him. This affect is called Cruelty, especially if it is believed that the one who loves has given no ordinary cause for Hatred.