Aristotle has just divided dialectic arguments into two categories: inductive and deductive. He continues his discussion of the sources of argument by claiming that there are four tools required to gain arguments. He does this in Topics, Book 1.
The classes, then, of things about which, and of things out of which, arguments are constructed, are to be distinguished in the way we have said before. The instruments whereby we are to become well supplied with deductions are four: one, the securing of propositions- second, the power to distinguish in how many ways an expression is used- third, the discovery of the differences of things- fourth, the investigation of likeness. The last three, as well, are in a certain sense propositions- for it is possible to make a proposition corresponding to each of them, e.g. that the desirable is either the honourable or the pleasant or the expedient- and that sensation differs from knowledge in that the latter may be recovered again after it has been lost, while the former cannot- and that the relation of the healthy to health is like that of the vigorous to vigour. The first proposition depends upon the use of one term in several ways, the second upon the differences of things, the third upon their likenesses.
What arguments are made from and what kinds of arguments there are Aristotle has just finished discussing. Now there are four tools that will give us as many deductive arguments as we need. First, how to gain new propositions. Second, the skill of determining how many different ways an expression is used. Third, how to discover the differences in things. Fourth, how to discover that two things are the same. These last three tools can be stated as propositions because we can make propositions that say the same thing. For example, the proposition what is desirable is either the same as what is pleasant, easy or honorable states the different ways that desirable can mean either pleasant, easy or honorable. The third tool can be formed into a proposition too. Sensation is different from knowledge because we can relearn what we have forgotten, but we cannot see again what have seen in the past is a proposition that describes the third tool. Finally, healthiness is to health like strength is to power describes the fourth tool.
Next, Aristotle discusses the first tool. He discusses where to find propositions to use in arguments.