Piety

Hosiotēs (ὁσιότης), with to hosion, 'the holy', names piety or religious righteousness — the right conduct owed to the gods, and more broadly the sacred rightness the gods approve. It was the heart of Greek religious life: a matter of proper ritual, reverence, and observance.

The Euthyphro question

Plato's Euthyphro turns this everyday notion into one of philosophy's most famous puzzles. As Euthyphro, on his way to prosecute his own father for impiety, claims to know exactly what holiness is, Socrates poses the question that still bears the dialogue's name: is the holy holy because the gods love it, or do the gods love it because it is holy?

The dilemma

Each horn cuts deep. If a thing is holy merely because the gods love it, then holiness is arbitrary, and divine approval can be no reason for it. But if the gods love it because it is already holy, then holiness is a standard independent of the gods, to which even they answer — and their approval cannot be what holiness is. The Euthyphro dilemma has framed every later debate about whether morality depends on the divine will, and the dialogue ends, fittingly, in aporia.