Aristotle has just finished discussing where to find differences. Now he continues his discussion of the tools of argument by showing where we should look for similarities. He does this in Topics, Book 1.
Likeness should be studied, first, in the case of things belonging to different genera, the formula being: as one is to one thing, so is another to another (e.g. as knowledge stands to the object of knowledge, so is perception related to the object of perception), or: as one is in one thing, so is another in another (e.g. as sight is in the eye, so is intellect in the soul, and as is a calm in the sea, so is windlessness in the air). Practice is more especially needed in regard to terms that are far apart- for in the case of the rest, we shall be more easily able to see the points of likeness. We should also look at things which belong to the same genus, to see if any identical attribute belongs to them all, e.g. to a man and a horse and a dog- for in so far as they have any identical attribute, in so far they are alike.
If we are examining justice, for example, then we should examine how justice is the same as something belonging to a different genus. Justice is like acting justly in the same way that knowledge is like learning. We could also use knowledge. Knowledge is like the thing we know in the same way that seeing is like the thing we see. Or seeing is in eyes in the same way that thinking is in the mind, calmness is in the sea and there is no wind in the air. When the things we find similarities in are very different, we will need practice to find out what is the same. From there, we should try to find if other things belonging to the same genus have any identical attributes. For example, do people, horses and dogs have any of the same attributes? If they are the same in any way, then they do have the same attributes.
Next, Aristotle examines the reason we use the second tool.