Now a dialectical proposition consists in asking something that is reputable to all men or to most men or to the wise, i.e. either to all, or to most, or to the most notable of these, provided it is not paradoxical- for a man would probably assent to the view of the wise, if it be not contrary to the opinions of most men. Dialectical propositions also include views which are like those which are reputable- also propositions which contradict the contraries of opinions that are taken to be reputable, and also all opinions that are in accordance with the recognized arts. Thus, supposing it to be reputable that the knowledge of contraries is the same, it might probably pass for reputable also that the perception of contraries is the same- also, supposing it to be a reputable opinion that there is but one single science of grammar, it might pass for a reputable opinion that there is but one science of flute-playing as well and if more than one science of grammar, more than one science of flute-playing as well- for all these seem to be alike and akin. Likewise, also, propositions contradicting the contraries of reputable opinions will pass as reputable- for if it is a reputable opinion that one ought to do good to ones friends, it will also be a reputable opinion that one ought not to do them harm. Here, that one ought to do harm to one’s friends is the contrary, and that one ought not to do them harm is the contradictory of that contrary.
A dialectical proposition asks something that everyone believes, most people believe or experts believe. Lets call opinions of this kind reputable. This does not include paradoxical things that some experts say. People will probably agree with experts as long as the expert does not say something that most people believe is false. Dialectical propositions also include propositions that are against the complete opposite of reputable opinions. They also include propositions that come from any recognized area of knowledge. A brief explanation of contraries and contradictories is necessary at this point. The contradictory of a proposition is simply disagreeing with it. So the contradictory of there was one man at the stadium is there was not one man at the stadium. The contrary of a proposition is as far opposite from it as it is possible to go. So the contrary proposition of there was one man at the stadium is there were many men at the stadium.
Suppose that the opinion knowledge of opposites is the same is reputable. In that case, it is probably reputable that the opinion perception of contraries is the same is reputable too. Another example is that if grammar is a single area of knowledge is a reputable opinion then flute-playing is a single area of knowledge is a reputable opinion too. Both of these start from reputable opinions and get similar opinions to them. We can also get reputable opinions from the contradictory of opinions contrary to reputable opinions. For example, if the proposition we should do good to our friends is reputable then so is we should not harm our friends. Harming is as far as it is possible to go from doing good, so a denial of that is reputable.
This discussion divides dialectic propositions into four categories. First, those that most people, all people or experts believe. Second, those propositions similar to ones all people, most people or experts believe. Third, those opinions that come from any recognized area of knowledge. Fourth, any opinion that is the contradictory to the contrary of any of the three previous kinds of opinions. All of these propositions are dialectical.
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.