P35: If someone imagines that a thing he loves is united with another by as close, or by a closer, bond of Friendship than that with which he himself, alone, possessed the thing, he will be affected with Hate toward the thing he loves, and will envy the other.

Dem.: The greater the love with which someone imagines a thing he loves to be affected toward him, the more he will exalt at being esteemed (by P34), i.e. (by P30 S), the more he will rejoice. And so (by P28) he will strive, as far as he can, to imagine the thing he loves to be bound to him as closely as possible. This striving, or appetite, is encouraged if he imagines another to desire the same thing he does (by P31). But this striving, or appetite, is supposed to be restrained by the image of the thing he loves, accompanied by the image of him with whom the thing he loves is united. So (by P11 S) he will thereby be affected with Sadness, accompanied by the idea of the thing he loves as a cause, together with the image of the other; i.e. (by P13 S), he will be affected with hate toward the thing he loves, and, at the same time, toward the other (by P15 C), whom he will envy because of the pleasure the other takes in the thing he loves (by P23), q.e.d.

Schol.: This Hatred toward a thing we love, combined with Envy, is called Jealousy, which is therefore nothing but a vacillation of mind born of Love and Hatred together, accompanied by the idea of another who is envied. Moreover, this hatred toward the thing he loves will be greater in proportion to the Joy with which the Jealous man was usually affected from the Love returned to him by the thing he loves, and also in proportion to the affect with which he was affected toward him with whom he imagines the thing he loves to unite itself. For if he hates him, he will thereby hate the thing he loves (by P24), because he imagines that what he loves affects with Joy what he hates, and also (by P15 C) because he is forced to join the image of the thing he loves to the image of him he hates.

This latter reason is found, for the most part, in Love toward a woman. For he who imagines that a woman he loves prostitutes herself to another not only will be saddened, because his own appetite is restrained, but also will be repelled by her, because he is forced to join the image of the thing he loves to the shameful parts and excretions of the other. To this, finally, is added the fact that she no longer receives the Jealous man with the same countenance as she used to offer him. From this cause, too, the lover is saddened, as I shall show.