On Benefits is a long essay, in seven books addressed to Seneca's friend Aebutius Liberalis, on how people ought to give, receive, and repay kindness. Seneca opens by naming ingratitude as one of the faults most hurtful to society, and traces it partly to our own carelessness: we scatter benefits without judgement, then resent that they are not returned. His aim is to teach the whole exchange, from the giver's first impulse to the receiver's lasting sense of debt.
His central claim is that a benefit does not consist in the thing handed over. What is paid or transferred is only the trace of a benefit; the benefit itself lives in the mind of the giver. The spirit in which a gift is offered can exalt small things and cheapen great ones, so an honest gift of oneself, like the poor pupil Aeschines offering Socrates nothing but himself, can outweigh the lavish presents of the rich.
Because the value lies in the will, Seneca insists that we give for the sake of giving. A good man does not write his gifts in a ledger or chase repayment to the day and hour. He bestows benefits as the gods do, helping even the unworthy, and treats the good deed itself as the reward. The risk of meeting an ungrateful person is no reason to stop, since the one who refuses to give sins earlier than the one who fails to repay.
Seneca then turns to the receiver. Gratitude is not the same as repayment: to accept a kindness gladly and acknowledge it is already to begin returning it. He examines why people fail at this, naming self-love, greed, and jealousy, and he counsels both giver and receiver on manner. A benefit given late or grudgingly is half spoiled, while one offered promptly and warmly, before a friend is forced to ask, gains enormously in worth.
The later books widen the argument. Seneca defends giving and gratitude as goods desirable for their own sake against those who would reduce virtue to profit, and in the closing book he asks how one ought to bear with the ungrateful: calmly, gently, and without regret, continuing to give even where past gifts were wasted. The work ends on its firmest note, that the true test of a great mind is to throw away one's bounty and still to give.