The normal method I will follow when presenting Aristotles thought is as follows. First, I will tell you where we are in Aristotles argument. Second, I will quote Aristotle himself from his book Topics. Third, I will explain what he said. Fourth, I will explain any problems or differences of opinion that exist. If you follow the links at the end, you will reach the next commentary. This commentary begins with Book 1. No previous knowledge of Aristotle is necessary at this point.
Our treatise proposes to find a line of inquiry whereby we shall be able to reason from reputable opinions about any subject presented to us, and also shall ourselves, when putting forward an argument, avoid saying anything contrary to it. First, then, we must say what deduction is, and what its varieties are, in order to grasp dialectical deduction- for this is the object of our search in the treatise before us.
This is the an introduction to the Topics themselves. Aristotle is claiming that his book Topics will teach us a particular kind of skill. It will teach us how to argue using opinions that everyone agrees on and argue without contradicting ourselves. In order to learn this skill, we need to know what we will be learning. Aristotle calls this particular skill dialectical deduction. So in order to learn this new skill we first have to know what deduction is, and what the various forms of deduction are.