It follows, then, that the happy person has the [stability] we are looking for and keeps the character he has throughout his life. For always, or more than anything else, he will do and study the actions in accord with virtue, and will bear fortunes most finely, in every way and in all conditions, since he is truly ‘good, foursquare, and blameless.
Many events, however, are subject to fortune; some are minor, some major. Hence, minor strokes of good or ill fortune clearly will not carry any weight for his life. But many major strokes of good fortune will make it more blessed; for in themselves they naturally add adornment to it, and his use of them proves to be fine and excellent. Conversely, if he suffers many major misfortunes, they oppress and spoil his blessedness, since they involve pain and impede many activities. And yet, even here what is fine shines through, whenever someone bears many severe misfortunes with good temper, not because he feels no distress, but because he is noble and magnanimous.
And since it is activities that control life, as we said, no blessed person could ever become miserable, since he will never do hateful and base actions. For a truly good and prudent person, we suppose, will bear strokes of fortune suitably, and from his resources at any time will do the finest actions, just as a good general will make the best use of his forces in war, and a good shoemaker will make the finest shoe from the hides given to him, and similarly for all other craftsmen.
If this is so, the happy person could never become miserable, but neither will he be blessed if he falls into misfortunes as bad as Priam’s. Nor, however, will he be inconstant and prone to fluctuate, since he will neither be easily shaken from his happiness nor shaken by just any misfortunes. He will be shaken from it, though, by many serious misfortunes, and from these a return to happiness will take no short time. At best, it will take a long and complete length of time that includes great and fine successes.
from Nicomachean Ethics, Trans. Terence Irwin, (Hackett Publishing, 1999), Bk 1, Ch 10: §11-14.