[14] But first we must notice that the word ‟criterion” is used both of that by which, as they say, we judge of reality and non-reality, and of that which we use as the guide of life; and our present task is to discuss the so-called criterion of truth, since we have already dealt with the criterion in its other sense in our discourse ‟On Scepticism.” ¹

[15] The criterion, then, with which our argument is concerned, has three several meanings—the general, the special, and the most special. In the ‟general” sense it is used of every standard of apprehension, and in this sense we speak even of physical organs, such as sight, as criteria. In the ‟special” sense it includes every technical standard of apprehension, such as the rule and compass. In the ‟most special” sense it includes every technical standard of apprehension of a non-evident object; but in this application ordinary standards ² are not regarded as criteria but only logical standards and those which the Dogmatists employ for the judging of truth.

[16] We propose, therefore, in the first place to discuss the logical criterion. But the logical criterion also may be used in three senses ³ —of the agent, or the instrument, or the ‟according to what;” the agent, for instance, may be a man, the instrument either sense-perception or intelligence, and the ‟according to what” the application of the impression ‟according to” which the man proceeds to judge by means of one of the aforesaid instruments. ⁴

[17] It was appropriate, I consider, to make these prefatory observations so that we may realize what is the exact subject of our discourse; and it remains for us to proceed to our counter-statement aimed against those who rashly assert that they have apprehended the criterion of truth, and we will begin with the dispute which exists about this question.