Stuart Kelly in The Guardian:
Friendship, like forgiveness, modesty and tolerance, is a concept which we all instinctively recognise but which buckles under the pressure of philosophical definition. In this little study, AC Grayling charts the history of attempts to understand what friendship is; how a friend differs from a lover, an acquaintance or an ally; and how friendship relates to wider moral and ethical propositions. Beginning with Plato and Aristotle, and progressing via Cicero and Augustine to Montaigne, Kant and Godwin, Grayling assesses a formidable array of sources before turning his attention to literary depictions of friendship: Achilles and Patroclus, David and Jonathan, Nisus and Euryalus, Tennyson and Hallam. He concludes with his own insights into the idea of friendship, drawn from his own experience.
…Friendship does have a political dimension – Aristotle said: “When men are friends there is no need for justice.” This idea was taken up by Jacques Derrida in The Politics of Friendship, a book absent from Grayling's bibliography. Derrida argues, to my mind convincingly, that the discourse around friendship has surreptitiously promoted it as a private, not public, virtue. There is a chasm between EM Forster's “If I had to choose between betraying my country and betraying my friend I hope I should have the guts to betray my country” and Carl Schmitt's notorious idea that “every totality of people looks for friends because it has already enemies”.