That which is variable includes that which man makes and that which man does: but making or production is different from doing or action (here we adopt the popular distinctions). The habit or formed faculty of acting with reason or calculation, then, is different from the formed faculty of producing with reason or calculation. And so the one cannot include the other; for action is not production, nor is production action.

Now, the builder’s faculty is one of the arts, and may be described as a certain formed faculty of producing with calculation; and there is no art which is not a faculty of this kind, nor is there any faculty of this kind which is not an art: an art, then, is the same thing as a formed faculty of producing with correct calculation.