There isn’t a logically watertight formal definition. And the fact that we can’t formally define friendship teaches us some important lessons about the nature of philosophical inquiry – lessons that we have (or should have) been learning ever since Plato, and that Wittgenstein helps to remind us of. But loosely and roughly, and without any promise of counter-example-proofness: friendship is benevolent companionship over time. Here what I mean by ‘companionship’ involves mutual second-personality: it means that I see someone else as a person who sees me as a person.