Aristotle, Nicomachean Ethics
H. Rackham, Ed.

because we are unwilling to call the living happy owing to the vicissitudes of fortune, and owing to our conception of happiness as something permanent and not readily subject to change, whereas the wheel of fortune often turns full circle in the same person's experience. [8] For it is clear that if we are to be guided by fortune, we shall often have to call the same man first happy and then miserable; we shall make out the happy man to be a sort of ‘chameleon, or a house built on the sand.’¹ [9]

But perhaps it is quite wrong to be guided in our judgement by the changes of fortune, since true prosperity and adversity do not depend on fortune's favours, although, as we said, our life does require these in addition; but it is the active exercise of our faculties in conformity with virtue that causes happiness, and the opposite activities its opposite. [10]

And the difficulty just discussed is a further confirmation of our definition; since none of man's functions possess the quality of permanence so fully as the activities in conformity with virtue: they appear to be more lasting even than our knowledge of particular sciences. And among these activities themselves those which are highest in the scale of values are the more lasting, because they most fully and continuously occupy the lives of the supremely happy: for this appears to be the reason why we do not forget them. [11]

The happy man therefore will possess that element of stability in question, and will remain happy all his life; since he will be always or at least most often employed in doing and contemplating