The same result is obtained from a special enquiry into the nature of the standard of truth. If the question is asked, How are true perceptions distinguished from false ones? the immediate reply given by the Stoics is, that a true perception is one which represents a real object as it really is. [158] You are no further with this answer, and the question has again to be asked, How may it be known that a perception faithfully represents a reality? The Stoics can only reply by pointing to a relative, but not to an absolute, test—the degree of strength with which certain perceptions force themselves on our notice. By itself a perception does not necessarily carry conviction or assent (συγκατάθεσις); for there can be no assent until the faculty of judgment is directed towards the perception, either for the purpose of allowing or of rejecting it, truth and error residing in judgment. Assent therefore, generally speaking, rests with us, as does also the power of decision; and a wise man differs from a fool quite as much by conviction as by action. [159] Some of our perceptions are, however, of such a kind that they at once oblige us to bestow on them assent, compelling us not only to regard them as probable, but also as true [160] and conformable to the actual nature of things. Such perceptions produce in us that strength of conviction which the Stoics call a conception; they are therefore termed conceptional perceptions. Whenever a perception forces itself upon us in this irresistible form, we are no longer dealing with a fiction of the imagination, but with something real; but whenever the strength of conviction is wanting, we cannot be sure of the truth of our perception. Or, expressing the same idea in the language of Stoicism, conceptional or irresistible perceptions, φαντασίαι καταληπτικαὶ, are the standard of truth. [161] The test of irresistibility [(γ) Primary conceptions a standard as well as irresistible perceptions.] (κατάληψις) was, in the first place, understood to apply to sensations from without, such sensations, according to the Stoic view, alone supplying the material for knowledge. An equal degree of certainty was, however, attached to terms deduced from originally true data, either by the universal and natural exercise of thought, or by scientific processes of proof. Now, since among these derivative terms some—the primary conceptions (κοιναὶ ἔννοιαι), for instance—serve as the basis for deriving others, it may in a certain sense be asserted that sensation and primary conceptions are both standards of truth. [162] In strict accuracy, neither sensation nor primary conceptions (πρόληψεις) can be called standards. The real standard, whereby the truth of a perception is ascertained, consists in the power, inherent in certain perceptions, of carrying conviction—τὸ καταληπτικὸν—a power which belongs, in the first place, to sensations, whether of objects without or within, and, in the next