Ethics, Part III, Proposition 42
He who has conferred a benefit on anyone from motives of love or honour will feel pain, if he sees that the benefit is received without gratitude.
Proof
When a man loves something similar to himself, he endeavours, as far as he can, to bring it about that he should be loved thereby in return (III. xxxiii.). Therefore he who has conferred a benefit confers it in obedience to the desire, which he feels of being loved in return; that is (III. xxxiv.) from the hope of honour or (III. xxx. note) pleasure; hence he will endeavour, as far as he can, to conceive this cause of honour, or to regard it as actually existing. But, by the hypothesis, he conceives something else, which excludes the existence of the said cause of honour: wherefore he will thereat feel pain (III. xix.). 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