Aristotle has just finished discussing how differences in genera can show a difference in meaning. He continues his discussion of the second tool by explaining how differences in definition can also do this. He does this in Topics, Book 1.
It is useful also to look at the definition that arises from the use of the term in combination, e.g. of a clear body and of a clear sound. For then if what is proper to each case be abstracted, the same phrase ought to remain over. This does not happen in the case of homonyms, e.g. in the cases just mentioned. For the former will be a body possessing such and such a colour, while the latter will be a sound easy to hear. Abstract, then, a body and a sound, and the remainder in each case is not the same. It should, however, have been, had clear in each case been synonymous.
Often in the actual accounts as well homonymy creeps in without being noticed, and for this reason the accounts also should be examined. If (e.g.) any one describes what betokens and produces health as being in a balanced state, we must not desist but go on to examine in what sense he has used the term balanced in each case, e.g. if in the latter case it means that it is of the right amount to produce health, whereas in the former it means that it is such as to betoken what kind of state prevails.
We can also tell if a word has multiple meanings by examining the definitions of the phrases that include that word. For example consider the definitions of -clear object and clear sound. A clear object is an object that someone can see through. A clear sound is a sound that is easily heard. Once you have the definitions, remove object and sound from them. The remainder should be the same if the words have the same meaning. Since this is clearly not what happens, the word clear has multiple meanings.
Sometimes the definitions will be the same, but some differences in meaning have still crept in without being noticed. It is for this reason that we should also check the definitions. For example, suppose we define whatever brings or indicates health as a balanced state. In that case, we also ask what balanced means. Perhaps it means the right amount of something to cause health, but it might also mean something that indicates health.
Next, Aristotle will discuss comparisons.