Likeness should be considered upon things that belong to different genera, that is, as one thing is to some one thing, so is another to another. E.g. as knowledge stands to knowable, so stands perception to perceptible (they are both cognitive of their objects). And as one thing is in some one thing, so is another in another. E.g. as sight is in the eye, so is intellect in the soul (viz. capacities). And as calm is in the sea, so is stillness in the air (each of them is a rest).
Most of all, we should be practised in things that are far apart; for we shall be more easily able to grasp the likeness in the rest.
Moreover, we should also consider those things which are in the same genus, whether there is something common to them all. E.g. to man, and horse, and dog; for inasmuch as there is something common to them (being pedestrian is what is common to them), in this point they are alike.
Bibliography: Aristotle Topics (108a.07)
Translation – text editing: George Kotsalis