Aristotle has been describing what sort of arguments dialectic has. He previously discussed what propositions are dialectic. He continues this discussion here in Topics, Book 1 by discussing contraries further.
Likewise also, if one ought to do good to one’s friends, one ought not to do good to one’s enemies: this too is the contradictory of the contrary—the contrary being that one ought to do good to one’s enemies. Likewise, also, in other cases. Also, on comparison, it will look like a reputable opinion that the contrary predicate belongs to the contrary subject: e.g. if one ought to do good to one’s friends, one ought also to do evil to one’s enemies. (It might appear as if doing good to one’s friends were a contrary to doing evil to one’s enemies; but whether this actually is or is not so in reality will be stated in the course of the discussion of contraries. Clearly also, all opinions that are in accordance with the arts are dialectical propositions; for people are likely to assent to the views held by those who have made a study of these things, e.g. on a question of medicine they will agree with the doctor, and on a question of geometry with the geometrician; and likewise also in other cases.
If the opinion “we should do good to our friends” is reputable, then so the opinion that “we should not do good to our enemies”. This is true for all contradictories contrary to reputable opinions. This analysis does not allow us to assume certain things. We might assume that the contrary predicate of a subject is predicated of the contrary subject, that the opinion is just as reputable as the original. For example, it might seem that if ‘we should do good to our friends’ is reputable then ‘we should do evil to our enemies’ is also reputable. Doing evil is the contrary to doing good, and friends are contrary to enemies. Aristotle does not commit to this here, but says that we must look at his discussion of contraries that occurs in Book 2. Aristotle does not argue for the position that expert opinions are reputable. Most people would believe expert opinions anyways, and that is enough to make them reputable.
There is one objection that modern logic gives here. It seems that it is not always true that the contradictory of a contrary is true if the original statement is true. For example, if it is reputable that ‘white is not a color’ is not reputable that ‘black is a color’ simply because black is contrary to white. Resolution of this problem will have to wait until the discussion of contraries in Topics, Book 2.
Next, Aristotle discusses dialectical problems.