Aristotle has just finished discussing the ten categories and how they apply to the fourfold division of dialectic that he had made earlier. Now he begins the new topic of obtaining good arguments. He begins this discussion in Topics, Book 1.
First, then, a definition must be given of a dialectical proposition and a dialectical problem. For it is not every proposition nor yet every problem that is to be set down as dialectical- for no one in his senses would make a proposition of what no one holds, nor yet make a problem of what is obvious to everybody- for the latter admits of no doubt, while to the former no one would assent.
The first step is giving a definition of a dialectical proposition and a dialectical problem. Not every proposition or problem is dialectical. No sane person would argue for a position that no one believes in nor would they argue against something that absolutely everyone agrees with. A problem must have reasonable doubt, and in neither of those cases does it exist. If no one believes in the position then such a position is unreasonable. If everyone believes that a position is wrong, then arguing for it is unreasonable.
This general truth will be used a bit later to discuss what dialectical problems are. This is also something that that modern philosophers agree with. We should not argue for the obvious, nor should we argue against the obvious. The reason is that any argument will not convince someone because it is not more obvious than the reasons they already have to believe what they believe.
Next, Aristotle discusses what dialectical propositions are.