divisions, has so little philosophical value, that it need not detain attention longer. [125] Two parts only of the Stoic logic possess any real interest—the theory of knowledge, and that part of dialectic which treats of ideas, and which in the main agrees with our formal logic. [B. Theory of knowledge.] [(1) General character of this theory.] The Stoic theory of knowledge turns about the enquiry for a criterion or standard by which what is true in notions may be distinguished from what is false. Since every kind of knowledge, no matter what be its object, must be tested by this standard, it follows that the standard cannot be sought in the subject-matter of notions, but, on the contrary, in their form. The enquiry after a standard becomes therefore identical with another—the enquiry as to what kind of notions supply a knowledge that may be depended upon, or what activity of the power of forming conceptions carries with it a pledge of its own truth. It is impossible to answer these questions without investigating the origin, the various kinds, and the value and importance of notions. Hence the problem proposed to the Stoics is reduced to seeking by an analysis of notions to obtain a universally valid standard by which their truth may be tested. Whether this enquiry was pursued by the older Stoics in all its comprehensiveness is a point on which we have no information. Boëthus, whose views on this subject were attacked by Chrysippus, had assumed the existence of several standards, such as Reason, Perception, Desire, Knowledge. Others, in the vaguest manner, had spoken of Right Reason (ὀρθὸς λόγος) as being the standard of truth. [126] Hence it may be inferred that before the time of Chrysippus the Stoics had no distinctly developed theory of knowledge. Nevertheless there are expressions of Zeno and Cleanthes still extant which prove that the essential parts of the later theory were already held by these philosophers, [127] although it is no doubt true that it first received that scientific form in which alone it is known to us at the hands of Chrysippus. [(2) Prominent points in the theory of knowledge.] The character of this theory of knowledge appears mainly in three particulars:—(1) In the importance attached by the Stoics to the impressions of the senses. This feature they inherited from the Cynics and shared with the Epicureans. (2) In the exaltation of expression into a conception—a trait distinguishing this from either of the two other contemporary Schools. (3) In the practical turn given to the question of a criterion or standard of truth. We proceed to the expansion of this theory in detail. [(a) Perceptions the result of impressions from without.]