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.