[Meaning] practice.
Dialectic is useful for three reasons: for exercise, for conversations, for philosophical sciences. Exercise, he is saying,[1] is what happens in discussion over things; for they take a problem and will make trial of it in disputations to assist them, making attacks through reputable opinions.[2] Or he might be saying that exercise is for each to attack in turn. Arguments of this sort were familiar to the ancients, and they had most conversations in this manner, not in books as now [for there were not that sort of books then], but with whatever situation established, they practiced and argued the inquiry of these things in conversations, constructing and demolishing the position through reputable opinions. And there are such books written by both Aristotle and Theophrastus having the attack against antithetical positions through reputable opinions. He is of course saying that dialectic is useful for this sort of exercise. For having some inquiring investigation of proofs (this is the knowledge of topics) we may be able to make an easy trial. For being able to divide an exercise in rhetoric and seeing at the same time both the problems and the arrangement of the subjects, they make an easy trial. For knowing the system in the dialectics they prosper at making an easy trial of the proposed subject. This sort of exercise is useful, that which is in accordance with the arguments and for the inquiry of what is sought and of the truth: for it prepares the mind for philosophy. For as the exercise of the body preserves the good health of the body according to a skill, so too the exercise of the mind preserves the proper good health of the mind by means of arguments and according to a method. The power of the rational mind is proper health, by which health it becomes critical and inquisitive of truth. It provides a second type of advantage for conversations.
Look under the letter "e" for "conversations" [enteuxeis].[3]