Aristotle has just finished explaining the reasons that we discover the various meanings of a word. Now he continues his discussion of the tools of argument by talking about the reasons that we find differences in things. He does this in Topics, Book 1.
The discovery of differences helps us both in deductions about sameness and difference, and also in recognizing what any particular thing is. That it helps us in deductions about sameness and difference is clear; for when we have discovered a difference of any kind whatever between the objects before us, we shall already have proved that they are not the same; while it helps us in recognizing what a thing is, because we usually distinguish the account that is proper to the substance of each particular thing by means of the differentiae that are appropriate to it.
Finding out various differences helps us to determine what is the same about two things and what is different. It also helps us to find out what a thing is. It helps us determine what is the same and what is different about two things by showing us that the two things are not the same. It helps us determine what kind of thing something is by explaining the differentia. Remember that the differentia is what makes that thing different from other things in the same genus. So by knowing the differences between that thing and other things in the same genus, we know what makes it different from the other things in the same genus.
Next, Aristotle discusses how finding similarities helps us with inductive arguments and hypothetical arguments.