Aristotle, Eudemian Ethics

But¹ one may raise the question whether it is possible to use any given thing both for its natural purpose and otherwise, and in the latter case to use it qua itself or on the contrary incidentally: for instance, with an eye qua eye, to see, or also just to see wrong, by squinting so that one object appears two—both these uses of the eye, then, use it because it is an eye, but it would be possible to make use of an eye but to use it in another way, incidentally, for example, if it were possible to sell it or to eat it. And similarly with the use of knowledge: one can use it truly, and one can use it wrongly—for instance, when one spells a word incorrectly on purpose, then at the time one is using knowledge as ignorance, just as dancing-girls sometimes interchange the hand and the foot and use foot as hand and hand as foot.² If then all the virtues are forms of knowledge, it would be possible to use even justice as injustice—in that case a man will be behaving unjustly by doing unjust acts as a result of justice, as when one makes ignorant mistakes from knowledge; but if this is impossible, it is clear that the virtues cannot be branches of knowledge.