QuestionsWhat is love?To love is to wish good to someone
What is love?Willing the other's good

To love is to wish good to someone

Love of friendship and love of concupiscence; amare est velle alicui bonum

Thomas Aquinas 1225 · 1274Faith and reason cannot contradict each other.
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What it claims

Love in the full sense is the love of friendship: to wish good to someone for his own sake.

Why

To love is to wish good to someone; loving a thing for oneself depends on loving a person for her own sake.

The same idea through history
Aristotle330 BCAristotle (Rhetoric, II, 4): to love is to wish for someone what one thinks good, for his sake and not one's own. Thomas takes up the formula as it stands and applies it to all love.
Thomas Aquinas1271Summa theologiae, Prima secundae, question 26, article 4: Thomas shows that love divides into love of friendship (one wishes good to someone) and love of concupiscence (one wishes a good for oneself or another), and that the first is love in the full sense.
In short

"To love is to wish good to someone" is an argument by Thomas Aquinas (Summa theologiae, I-II, q. 26, art. 4, 1271) on the question "What is love?" (answer: willing the other's good). Its claim: Love in the full sense is the love of friendship: to wish good to someone for his own sake. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Summa theologiae, I-II, q. 26, art. 4
Valid with an implicit premise

When we say "I love this dish" and "I love my sister", we mean two very different things. Thomas Aquinas shows it from a sentence of Aristotle: to love is to wish good to someone. Love therefore has two objects: the good one wishes, and the one to whom one wishes it. Loving someone for himself is love of friendship. Loving a thing for another, for example wine for the pleasure one draws from it, is love of concupiscence. The second depends on the first, as what exists in another depends on what exists by itself. Love in the full sense is therefore to wish someone's good for his own sake.

P1To love is to wish good to someone (Aristotle).
P2Love therefore tends to two objects: the good one wishes to someone (oneself or another), and the one to whom one wishes this good.
P3What is loved with the love of friendship is loved simply and for itself; what is loved with the love of concupiscence is loved for another. The second depends on the first, as relative being depends on being simply.
P4 · implicit premiseThe love that is not relative to another love is love properly speaking: the love that wishes someone's good for his own sake.
Love in the full sense is the love of friendship: to wish good to someone for his own sake.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P4, the computer finds a world where both loves exist, friendship depending on concupiscence, and where love in the full sense is the desire to possess. The other premises are true, the conclusion is false.
Valid with P4. Without it, two kinds of love are distinguished, one depending on the other, but nothing says which deserves the name of love. P4 takes up Thomas's idea: what is first and depends on nothing is the proper sense of the word, the other senses being derived. This is the premise contested by those who make lack or desire the heart of love.
Analogy. We love honey for its taste, and we love the beekeeper for herself. In the first case, the person who enjoys the honey is the real aim; in the second, she is the aim. Thomas says: the first love is a road to a good, the second is the good. His critics answer that we often desire someone only because he is missing to us.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Plato 380 BC · attacks premise 1

In the Symposium, Diotima says that whoever desires desires what is not provided or present, what he has not or lacks. Love is therefore first a lack, the desire to possess the good for ever, and not the gift of oneself to another.

Thomas d'Aquin replies
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Thomas Aquinas 1271 · Thomas Aquinas (q. 26, art. 4, text not directly addressing Plato)
Thomas does not deny this movement: he calls it love of concupiscence, by which a thing is loved that it may be the good of another or of oneself. But this love is a relative love: the love by which someone is loved that he may have good is love simply.
The debate goes on (our summary) This passage does not answer Plato, whom Thomas does not cite here. A Platonist answers that loving the other for himself already presupposes being drawn by what is good in him, hence a lack. A Thomist answers that lack explains how love begins, not what it is when complete.
Where the debate stands (our summary)The disagreement is about order: does love arise from a lack (Plato) or from a will for the good (Thomas)? Both recognise the two movements; they do not agree on which one defines love.
François de La Rochefoucauld 1665 · attacks premise 1

For La Rochefoucauld, self-love is "the love of self, and of all things for self". It is never quiet when out of itself and rests on other subjects only as a bee on flowers, to extract its proper food. What we take for wishing another's good would be disguised interest.

a defender replies
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Joseph Butler defender of the argument · 1726
Butler argues, in his Sermons, that self-love and benevolence are two distinct affections: we can want another's good for itself, and the satisfaction we draw from it presupposes that we first wanted it.
Where the debate stands (our summary)This debate on psychological egoism is not settled: philosophers and psychologists still discuss what experiments on empathy and altruism prove. Thomas only holds that one can wish another's good; he does not prove that everyone does.
Jean-Paul Sartre 1943 · attacks premise 4

For Sartre, love is by essence a deception: to love is to want to be loved, and so to want the other to want me to love him. The lover wants to possess a freedom as freedom. He does not want the other's good for itself; he wants to be loved by him.

no reply
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No reply possible: objection raised in 1943, after the author's death (1274).
The debate goes on (our summary) A Thomist can answer that Sartre describes passionate love and its traps, not the love of friendship in which one wants the other to be and to grow for himself. Beauvoir, in The Second Sex, admits that authentic love remains possible, based on the reciprocal recognition of two freedoms. Sartre would answer that this love remains to be built beyond the description.
Where the debate stands (our summary)The debate remains open: is the desire to be loved at the heart of all love (Sartre), or only of passionate love, friendship going beyond it (Thomas)? Sartre himself leaves the door open to an ethics of salvation, without describing it.