Lives and Opinions of Eminent Philosophers

​They say that the study of syllogisms is most useful; for it reveals what is demonstrative, which contributes greatly to the correction of doctrines, and scientific grasp produces order and memory. And an argument itself is a compound formed of premisses and a conclusion; a syllogism is a syllogistic argument composed of these; and a demonstration is an argument that, through what is more fully comprehended, concludes what is less comprehended. Impression is an imprint in the soul, the name having been appropriately transferred from the imprints produced in wax by a seal-ring.

​Of impression, one kind is cognitive and the other non-cognitive. The cognitive, which they say is the criterion of things, is that which arises from what exists and is imprinted and stamped in accordance with that very existing thing; while the non-cognitive is that which does not arise from what exists, or, though arising from what exists, is not in accordance with that very existing thing, being neither clear nor distinct. Dialectic itself is necessary and is a virtue containing virtues as species: non-precipitancy being the knowledge of when one ought to assent and when not; freedom from carelessness being a powerful rational account against what is merely probable, so as not to yield to it;

​irrefutability being strength in argument, so as not to be led away by it to the opposite side; and freedom from vanity being a state that refers impressions to right reason. Knowledge itself they say is either secure comprehension or a state in the reception of impressions unshakeable by reason. And without dialectical study the wise man will not be infallible in argument; for the true and the false are discerned by it, and what is plausible and what is ambiguously expressed are distinguished; and apart from it there is no methodical way of questioning and answering.

​Precipitancy in assertions extends also to actions, so that those who have untrained impressions turn to disorder and recklessness. Nor will the wise man appear sharp, quick-witted, and generally formidable in discourse in any other way; for it belongs to the same person to converse and to reason correctly, and to the same person to discourse on the matters proposed and to answer what is asked, which belong to a man experienced in dialectic. In logic, then, these are their views in summary. And so that we may also speak in detail and present those doctrines of theirs that pertain to the introductory art, Diocles of Magnesia sets them down word for word in his Survey of the Philosophers, speaking as follows:

"The Stoics are pleased to put the account of impression and perception first, insofar as the criterion by which the truth of things is known is generically an impression, and insofar as the account of assent, comprehension, and thought, which takes precedence over the others, does not subsist without impression. For impression leads the way, and then the intellect, possessing the capacity for speech, expresses in discourse what it experiences under the influence of the impression.

​Impression and phantasm differ; for a phantasm is an appearance in the intellect such as occurs during sleep, whereas an impression is an imprint in the soul, that is to say an alteration, as Chrysippus posits in the second book of On the Soul. For one must not accept the imprint as like the mark of a seal, since it is impossible for many imprints to occur at the same time upon the same thing. And cognitive impression is conceived as that which is stamped, imprinted, and sealed from what exists in accordance with that very existing thing, of such a kind as could not arise from what does not exist."