and so on ad infinitum ; if out of things indemonstrable, then, whether all or some or only a single one of the steps are the subject of doubt, the whole is indemonstrable." If you think, they add, that there are some things which need no demonstration, yours must be a rare intellect, not to see that you must first have demonstration of the very fact that the things you refer to carry conviction in themselves. Nor must we prove that the elements are four from the fact that the elements are four. Besides, if we discredit particular demonstrations, we cannot accept the generalization from them. And in order that we may know that an argument constitutes a demonstration, we require a criterion ; but again, in order that we may know that it is a criterion we require a demonstration ; hence both the one and the other are incomprehensible, since each is referred to the other. How then are we to grasp the things which are uncertain, seeing that we know no demonstration ? For what we wish to ascertain is not whether things appear to be such and such, but whether they are so in their essence. They declared the dogmatic philosophers to be fools, observing that what is concluded ex hypothesi is properly described not as inquiry but assumption, and by reasoning of this kind one may even argue for impossibilities. As for those who think that we should not judge of truth from surrounding circumstances or legislate on the basis of what is found in nature, these men, they used to say, made themselves the measure of all things, and did not see that every phenomenon appears in a certain disposition and in a certain reciprocal relation to surrounding circumstances. Therefore we must affirm either that all