But no one deliberates about that which cannot be altered, nor about that which it is not in his power to do.

Now science, we saw, implies demonstration; but things whose principles or causes are variable do not admit of demonstration; for everything that depends upon these principles or causes is also variable; and, on the other hand, things that are necessarily determined do not admit of deliberation. It follows, therefore, that prudence cannot be either a science or an art: it cannot be a science, because the sphere of action is that which is alterable; it cannot be an art, because production is generically different from action.

It follows from all this that prudence is a formed faculty that apprehends truth by reasoning or calculation, and issues in action, in the domain of human good and ill; for while production has another end than itself, this is not so with action, since good action or well doing is itself the end.