Ethics, Part III, Proposition 45
If a man conceives, that anyone similar to himself hates anything also similar to himself, which he loves, he will hate that person.
Proof
The beloved object feels reciprocal hatred towards him who hates it (III. xl.); therefore the lover, in conceiving that anyone hates the beloved object, conceives the beloved thing as affected by hatred, in other words (III. xiii.), by pain; consequently he is himself affected by pain accompanied by the idea of the hater of the beloved thing as cause; that is, he will hate him who hates anything which he himself loves (III. xiii. note). Q.E.D.
Benedict de Spinoza, The Ethics (Ethica Ordine Geometrico Demonstrata), translated from the Latin by R. H. M. Elwes.
First published in The Chief Works of Benedict de Spinoza, vol. II (London: George Bell and Sons, 1884).
This translation is out of copyright in the United States, where it was published in 1884, and wherever the copyright term is the author's life plus 100 years or less; its translator died in 1892.
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Digital text: Project Gutenberg eBook #3800