Aristotle has just finished explaining what theses are. He continues the discussion on the various kinds of arguments by discussing the difference between a problem and a thesis. He does this in Topics, Book 1.
Now a thesis also is a problem, though a problem is not always a thesis, inasmuch as some problems are such that we have no opinion about them either way. That a thesis is a problem, is clear- for it follows of necessity from what has been said that either the mass of men disagree with the wise about the thesis, or that the one or the other class disagree among themselves, seeing that the thesis is a paradoxical belief. Practically all dialectical problems indeed are now called theses. But it should make no difference whichever description is used- for our object in thus distinguishing them has not been to create a terminology, but to recognize what differences actually exist between them.
Given what Aristotle has already said about problems and theses, we know that some problems are not theses but all theses are problems because there are some problems that we have no opinion on. We know that a thesis is a problem because either most people disagree with the experts who promote the thesis, or most people disagree with the ones who argue for it. This is true because a thesis is a seemingly contradictory or absurd belief. In Aristotles time, some people began to call all dialectical problems thesis. Perhaps this was because they talked a lot about ideas that sounded absurd. In any case, what really matters is that we describe this kind of belief. The point in using thesis and distinguishing it from a problem is to recognize that they are different. Aristotle is not trying to fill our minds with useless technical words.
Next, Aristotle discusses what sort of problems we should argue about.