Aristotle has just finished discussing the various ways that we can find out the different meanings of a word. Now Aristotle continues his explanation of the various tools of argument by talking about what things we should investigate to determine differences. He does this in Topics, Book 1.
Thus when a term is used in many ways, it may be investigated by these and like means. The differences which things present to each other should be examined both in the genera themselves (e.g. Wherein does justice differ from courage, and wisdom from temperance for all these belong to the same genus)- and also from one genus to another, provided they are not too far apart (e.g. Wherein does perception differ from knowledge?)- for in the case of genera that are very far apart, the differences are entirely obvious.
When we investigate the different ways to use a word, we can use the ways I have already discussed and any like them. The third tool was finding the differences between things. But there are only some things that we should investigate. If we are investigating justice, we should investigate the differences it has with other things in the same genus. For example, what makes justice different from courage. We should also investigate the differences between each thing in that genus. What is the difference between wisdom and temperance? We should also try to understand the differences between various genus with the one we are investigating. Justice is a virtue. So what is the difference between virtues and other characteristics? We should not do this if the differences are obvious. So investigating the difference between people and rocks is unnecessary.
Next, Aristotle discusses where to find similarities.