[18] Of those, then, who have treated of the criterion some have declared that a criterion exists—the Stoics, for example, and certain others—while by some its existence is denied, as by the Corinthian Xeniades, ¹ amongst others, and by Xenophanes ² of Colophon, who says—‟Over all things opinion bears sway;” while we have adopted suspension of judgement as to whether it does or does not exist.

[19] This dispute, ³ then, they will declare to be either capable or incapable of decision; and if they shall say it is incapable of decision they will be granting on the spot the propriety of suspension of judgement, while if they say it admits of decision, let them tell us whereby it is to be decided, since we have no accepted criterion, and do not even know, but are still inquiring, whether any criterion exists.

[20] Besides, in order to decide the dispute which has arisen about the criterion, we must possess an accepted criterion by which we shall be able to judge the dispute; and in order to possess an accepted criterion, the dispute about the criterion must first be decided. And when the argument thus reduces itself to a form of circular reasoning the discovery of the criterion becomes impracticable, since we do not allow them to adopt a criterion by assumption, while if they offer to judge the criterion by a criterion we force them to a regress ad infinitum. And furthermore, since demonstration requires a demonstrated criterion, while the criterion requires an approved demonstration, they are forced into circular reasoning.

[21] We suppose, then, that this is sufficient to expose the rashness of the Dogmatists in respect of their doctrine of the Criterion; but in order to enable us to confute them in detail, it will not be out of place to dwell at length upon this topic. We do not, however, desire to oppose their opinions about the criterion severally, one by one—for their controversy is endless, and to do so would necessarily involve us as well in a confused discussion,—but inasmuch as the criterion in question is three-fold (the agent, the instrument, and the ‟according to what”), we shall discuss each of these in turn and establish the non- apprehensibility of each, since in this way our exposition will be at once both methodical and complete. Let us begin with the agent; for the perplexity which attaches to this seems somehow to involve the rest as well.