They say that the most useful of these parts is the consideration of syllogisms; for that they show us what are the things which are capable of demonstration, and that contributes much to the formation of our judgment, and their arrangement and memory give a scientific character to our knowledge. They define reasoning to be a system composed of assumptions and conclusions: and syllogism is a syllogistic argument proceeding on them. Demonstration they define to be a method by which one proceeds from that which is more known to that which is less. Perception, again, is an impression produced on the mind, its name being appropriately borrowed from impressions on wax made by a seal; and perception they divide into comprehensible and incomprehensible: Comprehensible, which they call the criterion of facts, and which is produced by a real object, and is, therefore, at the same time conformable to that object; Incomprehensible, which has no relation to any real object, or else, if it has any such relation, does not correspond to it, being but a vague and indistinct representation.
Dialectics itself they pronounce to be a necessary science, and a virtue which comprehends several other virtues under its species. And the disposition not to take up one side of an argument hastily, they defined to be a knowledge by which we are taught when we ought to agree to a statement, and when we ought to withhold our agreement. Discretion they consider to be a powerful reason, having reference to what is becoming, so as to prevent our yielding to an irrelevant argument. Irrefutability they define to be a power in an argument, which prevents one from being drawn from it to its opposite. Freedom from vanity, according to them, is a habit which refers the perceptions back to right reason.
Again, they define knowledge itself as an assertion or safe comprehension, or habit, which, in the perception of what is seen, never deviates from the truth. And they say further, that without dialectic speculation, the wise man cannot be free from all error in his reasoning. For that that is what distinguishes what is true from what is false, and which easily detects those arguments which are only plausible, and those which depend upon an ambiguity of language. And without dialectics they say it is not possible to ask or answer questions correctly. They also add, that precipitation in denials extends to those things which are done, so that those who have not properly exercised their perceptions fall into irregularity and thoughtlessness. Again, without dialectics, the wise man cannot be acute, and ingenious, and wary, and altogether dangerous as an arguer. For that it belongs to the same man to speak correctly and to reason correctly, and to discuss properly those subjects which are proposed to him, and to answer readily whatever questions are put to him, all which qualities belong to a man who is skilful in dialectics. This then is a brief summary of their opinions on logic.
XXXVI. And, that we may also enter into some more minute details respecting them, we will subjoin what refers to what they call their introductory science, as it is stated by Diocles, of Magnesia, in his Excursion of Philosophers, where he speaks as follows, and we will give his account word for word.
The Stoics have chosen to treat, in the first place, of perception and sensation, because the criterion by which the truth of facts is ascertained is a kind of perception, and because the judgment which expresses the belief, and the comprehension, and the understanding of a thing, a judgment which precedes all others, cannot exist without perception. For perception leads the way; and then thought, finding vent in expressions, explains in words the feelings which it derives from perception. But there is a difference between φαντασία and φάντασμα. For φάντασμα is a conception of the intellect, such as takes place in sleep; but φαντασία is an impression, τύπωσις, produced on the mind, that is to say, an alteration, ἀλλοίωσις, as Chrysippus states in the twelfth book of his treatise on the Soul. For we must not take this impression to resemble that made by a seal, since it is impossible to conceive that there should be many impressions made at the same time on the same thing. But φαντασία is understood to be that which is impressed, and formed, and imprinted by a real object, according to a real object, in such a way as it could not be by any other than a real object; and, according to their ideas of the φαντασίαι, some are sensible, and some are not. Those they call sensible, which are derived by us from some one or more senses; and those they call not sensible, which emanate directly from the thought, as for instance, those which relate to incorporeal objects, or any others which are embraced by reason. Again, those which are sensible, are produced by a real object, which imposes itself on the intelligence, and compels its acquiescence; and there are also some others, which are simply apparent, mere shadows, which resemble those which are produced by real objects.
Again, these φαντασίαι are divided into rational and irrational; those which are rational belong to animals capable of reason; those which are irrational to animals destitute of reason. Those which are rational are thoughts; those which are irrational have no name; but are again subdivided into artificial and not artificial. At all events, an image is contemplated in a different light by a man skilful in art, from that in which it is viewed by a man ignorant of art.
By sensation, the Stoics understand a species of breath which proceeds from the dominant portion of the soul to the senses, whether it be a sensible perception, or an organic disposition, which, according to the notions of some of them, is crippled and vicious. They also call sensation the energy, or active exercise, of the sense. According to them, it is to sensation that we owe our comprehension of white and black, and rough and smooth: from reason, that we derive the notions which result from a demonstration, those for instance which have for their object the existence of Gods, and of Divine Providence. For all our thoughts are formed either by indirect perception, or by similarity, or analogy, or transposition, or combination, or opposition. By a direct perception, we perceive those things which are the objects of sense; by similarity, those which start from some point present to our senses; as, for instance, we form an idea of Socrates from his likeness. We draw our conclusions by analogy, adopting either an increased idea of the thing, as of Tityus, or the Cyclops; or a diminished idea, as of a pigmy. So, too, the idea of the centre of the world was one derived by analogy from what we perceived to be the case of the smaller spheres. We use transposition when we fancy eyes in a man’s breast; combination, when we take in the idea of a Centaur; opposition, when we turn our thoughts to death. Some ideas we also derive from comparison, for instance, from a comparison of words and places.
There is also nature; as by nature we comprehend what is just and good. And privation, when for instance, we form a notion of a man without hands. Such are the doctrines of the Stoics, on the subject of phantasia, and sensation, and thought.
XXXVII. They say that the proper criterion of truth is the comprehension, φαντασία; that is to say, one which is derived from a real object, as Chrysippus asserts in the twelfth book of his Physics; and he is followed by Antipater and Apollodorus. For Boethus leaves a great many criteria, such as intellect, sensation, appetite, and knowledge; but Chrysippus dissents from his view, and in the first book of his treatise on Reason, says, that sensation and preconception are the only criteria. And preconception is, according to him, a comprehensive physical notion of general principles. But others of the earlier Stoics admit right reason as one criterion of the truth; for instance, this is the opinion of Posidonius, and is advanced by him in his essay on Criteria.
XXXVIII. On the subject of logical speculation, there appears to be a great unanimity among the greater part of the Stoics, in beginning with the topic of the voice. Now voice is a percussion of the air; or, as Diogenes, the Babylonian, defines it, in his essay on the Voice, a sensation peculiar to the hearing. The voice of a beast is a mere percussion of the air by some impetus: but the voice of a man is articulate, and is emitted by intellect, as Diogenes lays it down, and is not brought to perfection in a shorter period than fourteen years. And the voice is a body according to the Stoics; for so it is laid down by Archedemus, in his book on the Voice, and by Diogenes, and Antipater, and also by Chrysippus, in the second volume of his Physics. For everything which makes anything, is a body; and the voice makes something when it proceeds to those who hear from those who speak.