Aristotle has finished explaining the goal of his book. Now he is going to describe the various kinds of propositions used in arguments. He does this in Topics, Book 1.
First, then, we must see of what parts our inquiry consists. Now if we were to grasp with reference to how many, and what kind of, things arguments take place, and with what materials they start, and how we are to become well supplied with these, we should have sufficiently won our goal. Now the materials with which arguments start are equal in number, and are identical, with the subjects on which deductions take place. For arguments start with propositions, while the subjects on which deductions take place are problems. Now every proposition and every problem indicates either a genus or a property or an accident for the differentia too, being generic, should be ranked together with the genus. Since, however, of what is proper to anything part signifies its essence, while part does not, let us divide the proper into both the aforesaid parts, and call that part which indicates the essence a definition, while of the remainder let us adopt the terminology which is generally current about these things, and speak of it as a property.
We must start by dividing the topic of dialectic into various parts. We should discuss how many kinds of arguments there are, how many arguments there are, what the arguments use and how to think up the right arguments for the right situations. If we do this, we will achieve our goal. Arguments start with the same number of subjects that you can deduce things from. Arguments start with propositions, and the subject of the argument is the problem. So both propositions and problems fall into three categories: genus, property and accident. The differentia belongs in the category of genus too. Property can be divided into two. Some properties are about what-something-is, and some are not. Lets call the part that is about the essence (what-it-is) a definition. We can call the other part a property.
Aristotle does not bother to explain these terms right away. Later on, he will explain what these various terms mean. By doing that, he will prove that all propositions and problems can be divided into these four categories. So it is probably best to see this as an outline of what Aristotle will have to prove later on.
Next Aristotle will explain the difference between a proposition and a problem.