Aristotle points out that similarities help us to construct inductive and hypothetical arguments. Now he continues his discussion of the tools of argument by showing that similarities help us construct definitions. He does this in Topics, Book 1.
It is useful for the rendering of definitions because, if we are able to see what is the same in each individual case of it, we shall be at no loss when we define it- for of the common predicates that which is most definitely predicated in what the thing is is likely to be the genus. Likewise, also, in the case of objects widely divergent, the examination of likeness is useful for purposes of definition, e.g. the sameness of a calm at sea, and windlessness in the air (each being a form of rest), and of a point on a line and the unit in number (each being a principle). If, then, we render as the genus what is common to all the cases, we shall get the credit of defining not inappropriately. Definition-mongers too nearly always render them in this way- for they declare the unit to be the principle of number, and the point the principle of a line. It is clear, then, that they place them in that which is common to both as their genus.
If we know what makes a individual of the same kind the same, then we will know what kind of thing it is. This makes constructing definitions easy. Remember that definitions just tell us what something is. The common predicates we use when we ask what something is are likely to be the genus. So if we are trying to define human, then we should note that human beings are a kind of animal (or primate). That would be the genus. Sometimes the things we are considering are very different from each other. In that case, we still use the same method. So the genus of calm in describing a calm sea and of a lack of wind in the air is rest, while the genus of point in a line and unit in a number is a principle. People who define things define them this way as well so it should be clear that it is the correct way of defining things.
There is only one problem with Aristotles method here. There may be more than one answer to the question what kind of thing is this. Finding out which answer is right is not dealt with here. Aristotle explains how to do that somewhere else. Remember that this book is about dialectic, not demonstration- we have to look to the part of the book on definitions to find an answer.
Next, Aristotle concludes Book 1 and introduces Book 2.