Chelidōn

Εἰ δὴ οὕτως, ἀνθρώπου δὲ τίθεμεν ἔργον ζωήν τινα, ταύτην δὲ ψυχῆς ἐνέργειαν καὶ πράξεις μετὰ λόγου, σπουδαίου δ’ ἀνδρὸς εὖ ταῦτα καὶ καλῶς, ἕκαστον δ’ εὖ κατὰ τὴν οἰκείαν ἀρετὴν ἀποτελεῖται· εἰ δὴ οὕτω, τὸ ἀνθρώπινον ἀγαθὸν ψυχῆς ἐνέργεια γίνεται κατ’ ἀρετήν, εἰ δὲ πλείους αἱ ἀρεταί, κατὰ τὴν ἀρίστην καὶ τελειοτάτην. ἔτι δ’ ἐν βίῳ τελείῳ· μία γὰρ χελιδὼν ἔαρ οὐ ποιεῖ, οὐδὲ μία ἡμέρα· οὕτω δὲ οὐδὲ μακάριον καὶ εὐδαίμονα μία ἡμέρα οὐδ ὀλίγος χρόνος.
(Aristotle, Eth. Nic. 1.7.14-16 = 1098a12-20)

If this is so, and if we declare that the function of man is a certain form of life, and define that form of life as the exercise of the soul’s faculties and activities in association with rational principle, and say that the function of a good man is to perform these activities well and rightly, and if a function is well performed when it is performed in accordance with its own proper excellence—from these premises it follows that the Good of man is the active exercise of his soul’s faculties in conformity with excellence or virtue, or if there be several human excellences or virtues, in conformity with the best and most perfect among them. Moreover, to be happy takes a complete lifetime; for one swallow does not make spring, nor does one fine day; and similarly one day or a brief period of happiness does not make a man supremely blessed and happy. (tr. Harris Rackham)