things. Again, if a person dividing (the problem) begs the thing proposed for discussion; as if when it is necessary to show that medicine belongs to the healthy, and the diseased, he should claim each of these, to be granted separately. Or if some one should beg one of these, which are necessarily consequent to each other, as that the side of a square is incommensurate with the diameter, when he ought to show, that the diameter is incommensurate with the side.
Contraries, are begged in as many ways, as the original question; for first, if any one should demand the opposites, affirmation and negation; secondly, contraries according to opposition, as that good and evil are the same; thirdly, if a man claiming universal to be granted should require contradiction particularly, as if assuming one science of contraries, he should desire it to be granted that there is different science of the wholesome and the unwholesome, or begging this, endeavoured to assume opposition as to the universal. Again, if a man should beg the contrary to what happens necessarily through the things laid down; if also, a person should not indeed assume the opposites themselves, but should claim two such things from which there will be an opposite contradiction. Still, there is a difference between assuming contraries and a petitio principii, because the error of the one belongs to the conclusion, (for having respect to this, we say that the original question is begged,) but contraries are in the propositions, from these subsisting in a certain way, as to each other.
Chapter 14
For the exercise and practice of such arguments as these, we must, in the first place, be accustomed to convert arguments; for thus we shall be better provided for the subject of discussion, and we shall obtain a knowledge of many argu-
