“Life seems to be common even to plants, but we are seeking what is peculiar to man. Let us exclude, therefore, the life of nutrition and growth. Next there would be a life of perception, but it also seems to be common even to the horse, the ox, and every animal. There remains, then, an active life of the element that has a rational principle; of this, one part has such a principle in the sense of being obedient to one, the other in the sense of possessing one and exercising thought. And, as "life of the rational element" also has two meanings, we must state that life in the sense of activity is what we mean; for this seems to be the more proper sense of the term. Now if the function of man is an activity of soul which follows or implies a rational principle, and if we say "so-and-so" and "a good so-and-so" have a function which is the same in kind, e. g. a lyre, and a good lyre-player, and so without qualification in all cases, eminence in respect of goodness being added to the name of the function (for the function of a lyre-player is to play the lyre, and that of a good lyre-player is to do so well): if this is the case, and we state the function of man to be a certain kind of life, and this to be an activity or actions of the soul implying a rational principle, and the function of a good man to be the good and noble performance of these, and if any action is well performed when it is performed in accordance with the appropriate excellence: if this is the case, human good turns out to be activity of soul in accordance with virtue, and if there are more than one virtue, in accordance with the best and most complete.
But we must add "in a complete life."”

For one swallow does not make a summer, nor does one day; and so too one day, or a short time, does not make a man blessed and happy.
Book I, 1098a; §7 as translated by W. D. Ross
Variants:
One swallow does not a summer make.
As quoted in A History of Ancient Philosophy: From the Beginning to Augustine (1998) by Karsten Friis Johansen, p. 382
One swallow (they say) no Sommer doth make.
John Davies, in The Scourge of Folly (1611)
One swallow yet did never summer make.
As rendered by William Painter in Chaucer Newly Painted (1623)
One swallow does not make a spring, nor does one sunny day; similarly, one day or a short time does not make a man blessed and happy.
As translated in Philosophical Grounds of Rationality: Intentions, Categories, Ends (1988), by Richard E. Grandy and ‎Richard Warner, p. 483
Nicomachean Ethics