Aristotle has just finished discussing how a difference in names indicates a difference in meaning. Now he will continue talking about the second tool by explaining how some differences in kind are obvious. He does this in Topics, Book 1.
In some cases there is no discrepancy of any sort in the names used, but a difference of kind is at once obvious: e.g. in the case of clear and obscure- for sound is called clear and obscure, just as colour is too. As regards the names, then, there is no discrepancy, but the difference in kind is at once obvious- for colour is not called clear in a like way to sound. This is plain also through sensation- for of things that are the same in kind we have the same sense, whereas we do not judge clearness by the same sense in the case of sound and of colour, but in the latter case we judge by sight, in the former by hearing. Likewise also with sharp and dull in regard to flavours and bodies: here in the latter case we judge by touch, but in the former by taste. For here again there is no discrepancy in the names used, in the case either of the original terms or of their contraries- for the contrary of sharp in either case is dull.
In some cases we cannot tell whether or not there is a difference in meaning by the names because the word and the contrary have only one name each. However, in some cases, the difference in meaning is obvious because there is a difference in kind. For example, the words clear and obscure can be applied to both sound and color. We know that clear sound and clear color do not mean the same things. This is even more obvious when we consider things. Sounds and colors are sensed by different senses, and if the words meant the same thing we would use the same sense to sense them. This is also true with the words sharp and dull. Sharp flavors are sensed with our taste buds, but sharp objects are sensed using touch. The contraries are still the same, but it is obvious that there is a difference in meaning.
Next, Aristotle discusses the presence of contraries as a way of determining difference in meaning.