expediency have no fixedness any more than matters of health. And if the subject in its general maxims is such, still less in its application to particular cases is exactness attainable:50 because these fall not under any art or system of rules, but it must be left in each instance to the individual agents to look to the exigencies of the particular case, as it is in the art of healing, or that of navigating a ship. Still, though the present subject is confessedly such, we must try and do what we can for it.
First then this must be noted, that it is the nature of such things to be spoiled by defect and excess; as we see in the case of health and strength (since for the illustration of things which cannot be seen we must use those that can), for excessive training impairs the strength as well as deficient: meat and drink, in like manner, in too great or too small quantities, impair the health: while in due proportion they cause, increase, and preserve it.