Telos

Telos (τέλος) means end — but in the rich sense of completion, fulfillment, or goal, not mere stopping. The verb telein means 'to complete' or 'to bring to fulfillment', and the word could even name the culmination of a sacred rite. It is the root of our 'teleology', the study of purposes, and of the prefix 'teleo-'.

Aristotle's final cause

The idea stands at the heart of Aristotle's philosophy of nature. Among his four causes, the telos is the final cause — 'that for the sake of which' a thing exists or acts. The acorn grows for the sake of becoming an oak; the eye exists for the sake of seeing. Nature, he held, does nothing in vain: each thing has an end built into what it is, and is fully itself only when it attains it.

The end of a human life

For ethics this means asking after the telos of human life as a whole — the final good for whose sake all else is chosen. Aristotle and the schools after him answered: eudaimonia, flourishing, the activity of the soul in accordance with excellence. To know a thing's telos is to know what it is for — and so what it is for it to be good of its kind.