Therefore, if reason, relying on things comprehended and perceived, and advancing in reliance on them, establishes the point that nothing can be comprehended, what can be found which can be more inconsistent with itself? And as the very nature of an accurate discourse professes that it will develop something which is not apparent, and that, in order the more easily to succeed in its object, it will employ the senses and those things which are evident, what sort of discourse is that which is uttered by those men who insist upon it that everything has not so much an existence as a mere appearance? But they are convicted most of all when they assume, as consistent with each other, these two propositions which are so utterly incompatible: first of all,—That there are some false perceptions;—and in asserting this they declare also that there are some which are true: and secondly, they add at the same time,—That there is no difference between true perceptions and false ones. But you assumed the first proposition as if there were some difference; and so the latter proposition is inconsistent with the former, and the former with the latter. But let us proceed further, and act so as in no respect to seem to be flattering ourselves; and let us follow up what is said by them, in such a manner as to allow nothing to be passed over. In the first place, then, that evidentness which we have mentioned has sufficiently great power of itself to point out to us the things which are just as they are. But still, in order that we may remain with firmness and constancy in our trust in what is evident, we have need of a greater degree of either skill or diligence, in order not, by some sort of juggling or trick, to be driven away from those things which are clear of themselves. For Epicurus, who wished to remedy those errors, which seem to perplex one’s knowledge of the truth, and who said that it was the duty of a wise man to separate opinion from evident knowledge, did no good at all; for he did not in the least remove the errors of opinion itself. XV. Wherefore, as there are two causes which oppose what is manifest and evident, it is necessary also to provide oneself with an equal number of aids. For this is the first obstacle, that men do not sufficiently exert and fix their minds upon those things which are evident, so as to be able to understand how great the light is with which they are surrounded. The second is, that some men, being deluded and deceived by fallacious and captious interrogatories, when they cannot clear them up, abandon the truth. It is right, therefore, for us to have those answers ready which may be given in defence of the evidentness of a thing,—and we have already spoken of them,—and to be armed, in order to be able to encounter the questions of those people, and to scatter their captious objections to the winds: and this is what I propose to do next.