place, to primary conceptions formed from them in a natural way—κοιναὶ ἔννοιαι or προλήψεις. On the other hand, conceptions and terms formed artificially can only have their truth established by being subjected to a scientific process of proof. How, after these statements, the Stoics could attribute a greater strength of conviction to artificial than to primary conceptions; [163] how they could raise doubts as to the trustworthiness of simple sensations, [164] is one of the paradoxes of the Stoic system, which prove the existence, as in so many other systems, of a double current of thought. There is, on the one hand, a seeking for what is innate and original, a going back to nature, an aversion to everything artificial and of human device, inherited by Stoicism from its ancestral Cynicism. On the other hand, there is a desire to supplement the Cynic appeal to nature by a higher culture, and to assign scientific reasons for truths which the Cynics laid down as self-evident. The latter tendency will alone explain the care and precision which the Stoics devoted to studying the forms and rules which govern intellectual processes. Attention to this branch of study may be noticed in Zeno and his immediate successors at the first separation of Stoicism from Cynicism. [165] Aristo is the only Stoic who is opposed to it, his whole habit of mind being purely that of a Cynic. In Chrysippus it attained its greatest development, and by Chrysippus the formal logic of the Stoics reached scientific completeness. In later times, when Stoicism reverted more nearly to its original Cynic type, and appealed directly to the immediate suggestions of the mind, it lost its interest in logic, as may be observed in Musonius, Epictetus, and others. For the present, however, let it suffice to consider the logic of Chrysippus, as far as that is known to us. [C. Formal logic.] [(1) Utterance in general.] The term formal logic is here used to express those investigations which the Stoics included under the doctrine of utterance. [166] The common object of those enquiries is that which is thought, or, as the Stoics called it, that which is uttered (λεκτόν), understanding thereby the substance of thought—thought regarded by itself as a distinct something, differing alike from the external object to which it refers, from the sound by which it is expressed, and from the power of mind which produces it. For this reason, they maintain that only utterance is not material; things are always material; even the process of thought consists in a material change within the soul, and an uttered word, in a certain movement of the atmosphere. [167] A question is here suggested in passing, which should not be lost sight of, viz. How far was it correct for the Stoics to speak of thoughts as existing, seeing they are not material, since, according to their teaching, reality only belongs to material things? [168]