§ 41.
1. The Stoics teach that all intellectual knowledge takes rise in sensuous perception. The soul, at first, is like a sheet of blank paper, on which representations of things are afterwards delineated by the senses. The beginning of all knowledge is, therefore, the aisthêsis (perception of sense). This, as soon as we are conscious of it, becomes a Representation (phantasia) or mental image. During the formation of this Representation the Soul is purely passive, the Representation is like the impression of a seal on wax (tupôsis en psuchê, for which Chrysippus, to modify the doctrine, substituted heteroiôsis psuchês, an alteration in the Soul). According to this view, the object of itself produces its Representation on the subject, and this Representation manifests itself, and in itself the corresponding object, to the subject. When we have apprehended an object, the remembrance of this object remains after the object has been removed. A large number of memories of this kind constitute experience (empeiria).
2. In the further progress of the process of thought, Concepts are formed from these Representations. The formation of Concepts is effected in two ways. Some are formed spontaneously and without conscious co-operation on our part (anepitechnêtôs). Others are the outcome of a deliberate and methodical process of thought. A number of similar Representations having been produced within us, there arise, spontaneously and without any reflex thought on our part, certain universal notions, which form a basis for the reflex and methodical formation of Concepts. These notions are called by the Stoics prolêpseis or koinai ennoiai. In a second stage, the reflex activity of thought is exercised. It detects resemblances and analogies, transforms and combines notions, and so forms artificially reflex Concepts, called by the Stoics ennoiai. For the ten categories of Aristotle the Stoics substitute, as ultimate universal concepts (genikôtata), Substance (or Substratum), Essential Quality, Accidental State or Condition, and Relation.
3. Judgment and Inference depend upon Concepts. The Stoics added to the theory of inference their doctrine regarding the hypothetical syllogism -- a form of reasoning which Aristotle did not specially investigate. By inference, say the Stoics, we are able to advance from one truth to another, and thus are in a position to investigate the causes of phenomena. In this way Science (epistêmê) is created -- the highest form of human knowledge. The right formation both of Concepts and of Judgments and Inferences is regulated by certain rules, which it is the province of Dialectic to lay down.
4. With regard to the relation subsisting between Concept and Being, the Stoics seem to have adopted the view which, in the Middle Ages, was known as that of the Nominalists. They combat alike the Platonic and the Aristotelian doctrine of the objective reality of Concepts; they assert that the Concept is something purely subjective, formed by a process of abstraction, to which, however, no real being corresponds in the objective order. The individual, as such, is the only thing which has real existence; the universal concept is a purely subjective product of the process of thought, whether we consider the form of the thought, or the thing given in the thought. In this doctrine we have distinctly brought before us the purely empirical character of the Stoic Theory of Knowledge. For in this theory Concepts are deprived of all relation to the essential being of things, and are thus reduced to mere generalized sensuous perceptions.
5. The Stoics, in their Theory of Knowledge, occupy themselves largely with the question of a criterion of truth. They find this criterion in the katalêpsis (Apprehension). This katalêpsis is attained when the object is represented in the mind with such clearness, force, and energy of conviction, that the truth of the representation cannot be denied. In such circumstances, the representation, and in the representation the object, is grasped or apprehended (katalambanetai) with absolute certainty. A representation thus clear, and thus forcing conviction (phantasia katalêptikê), is necessarily recognised as indubitably true, while the representation which does not exhibit this clearness or carry this force of conviction (phantasia akatalêptos) does not give the same certainty, and must, therefore, be regarded only as more or less probable.
6. In accordance with these principles, the Stoics define Knowledge as (Stob. Ecl. Eth. II. 128) katalêpsis asphalês kai ametaptôtos hupo logou -- certain and indisputable apprehension by means of a concept, and define Science as a system from such apprehensions. According to Cicero (Acad. II. 47), Zeno compared Perception to the extension of the fingers, Assent (sugkatathesis) to the hand half-closed, the Apprehension of the object (katalêpsis) to the hand fully closed (the fist), and Knowledge to the grasping of the fist by the other hand, whereby it is more strongly and securely closed. Knowledge, according to this account of the theory, is katalêpsis perfected. It is, however, to be remarked that on the point here in question the several Stoics differ widely from one another.