Aristotle, Nicomachean Ethics
H. Rackham, Ed.

But let it be granted to begin with that the whole theory of conduct is bound to be an outline only and not an exact system, in accordance with the rule we laid down at the beginning,¹ that philosophical theories must only be required to correspond to their subject matter; and matters of conduct and expediency have nothing fixed or invariable about them, any more than have matters of health. 2. [4] And if this is true of the general theory of ethics, still less is exact precision possible in dealing with particular cases of conduct; for these come under no science or professional tradition, but the agents themselves have to consider what is suited to the circumstances on each occasion, just as is the case with the art of medicine or of navigation. 2. [5] But although the discussion now proceeding is thus necessarily inexact, we must do our best to help it out.2. [6]

First of all then we have to observe, that moral qualities are so constituted as to be destroyed by excess and by deficiency—as we see is the case with bodily strength and health （for one is forced to explain what is invisible by means of visible illustrations）. Strength is destroyed both by excessive and by deficient exercises, and similarly health is destroyed both by too much and by too little food and drink; while they are produced, increased and preserved by suitable quantities. 2. [7] The same therefore is true of Temperance, Courage, and the other virtues.