Aristotle has just finished explaining where to find differences and similarities between things. He continues his discussion of the tools of argument by showing why finding the various meanings of a word is important. He does this in Topics, Book 1.
It is useful to have examined the number of uses of a term both for clearness sake (for a man is more likely to know what it is he asserts, if it has been made clear to him how many uses it may have), and also with a view to ensuring that our deductions shall be in accordance with the actual facts and not addressed merely to the word used. For as long as it is not clear in how many ways a term is used, it is possible that the answerer and the questioner are not directing their minds upon the same thing- whereas when once it has been made clear how many uses there are, and also upon which of them the former directs his mind when he makes his assertion, the questioner would then look ridiculous if he failed to address his argument to this. It helps us also both to avoid being misled and to mislead by fallacies- for if we know the number of uses of a term, we shall certainly never be misled by fallacy, but shall know if the questioner fails to address his argument to the same point- and when we ourselves put the questions we shall be able to mislead him, if our answerer happens not to know the number of uses of our terms. This, however, is not possible in all cases, but only when of the many uses some are true and others are false. This manner of argument, however, does not belong properly to dialectic- dialecticians should therefore by all means beware of this kind of verbal discussion, unless any one is absolutely unable to discuss the subject before him in any other way.
There are two reasons to examine the number of ways a word can be used. The first reason is that we need to be clear. If we are clear, then it is more likely we will know what we are arguing about. The second reason is that our deductions will be about the way things are in the world rather than about words. If we do not determine the various ways that words are used, then it is possible that we will not be arguing about the same thing as our opponent. When we explain what the word can mean and which meaning of the word we are talking about, then it is clear to the opponent and everyone else what we are talking about. If our opponent argued against something we did not say, he would look ridiculous. There is a fallacy that is arguing against something the other person did not say. By clearly understanding what meaning the various words have, we will not commit that fallacy.
It is also possible for us to use our knowledge of the various meanings of words to deceive our opponent. If some of the meanings of the words make our propositions true and other make them false, then by failing to distinguish between them we can lead our opponents into confusion. This kind of argument is not really dialectic it is a fallacy. Therefore, if we are using dialectic, we should not do this unless we have no choice at all.
Aristotles first fallacy is called straw man and the second fallacy is equivocation. These reasons for learning this kind of material are still used today. As for Aristotles last argument, we only really have no choice when we need to save someones life or something equally dire requires us to deceive others.
Next, Aristotle discusses why the examination of the differences in things is important.