Aristotle has just finished discussing how a difference in meaning is sometimes obvious. Now he will continue his discussion of the second tool by explaining how the presence or absence of a contrary shows difference in meaning. He does this in Topics, Book 1.
Moreover, see if one use of a term has a contrary, while another has absolutely none- e.g. the pleasure of drinking has a contrary in the pain of thirst, whereas the pleasure of seeing that the diagonal is incommensurate with the side has none, so that pleasure is used in more than one way. To love also, used of the frame of mind, has to hate as its contrary, while as used of the physical activity it has none- clearly, therefore, to love is homonymous.
We can also compare whether one use of a word has a contrary while another use does not. For example, pleasure of drinking is contrary to the pain of being thirsty, but the pleasure of seeing that the diagonal of a right angled triangle is never the same as any of the sides has no contrary. This shows that pleasure is used in more than one meaning. The word love is another example. Consider a person who loves rock climbing. The contrary of that is someone who hates rock climbing. But there is no contrary to someone who makes love to another person. Therefore, love also has multiple meanings.
Next, Aristotle discusses intermediates as a way of detecting differences in meaning.