We shall be in perfect possession of the way to proceed when we are in a position like that which we occupy in regard to rhetoric and medicine and faculties of that kind- this means the doing of that which we choose with the materials that are available. For it is not every method that the rhetorician will employ to persuade, or the doctor to heal: still, if he omits none of the available means, we shall say that his grasp of the science is adequate.
We have completely learned dialectic when we have learned it as well as any skill can learned such as medicine and rhetoric. We can do what we would like with the materials available. In dialectic, this means that we can oppose or commend any kind of argument using all of the relevant methods. We dont have to use every method in every situation, just the ones that are available given the situation. If we are able to do that, then our understanding of dialectic is adequate.
There are two issues to comment on here. First, Aristotle does not claim that our understanding of dialectic will not be helped by practice. It certainly will. However, the theoretical understanding of dialectic will not be improved by practicing. Second, there are certainly many kinds of specialized arguments. Aristotle tried to cover them all generally, but logic journals continue to exist and discuss such issues today. Not only that, but Aristotle does not even cover such arguments in this book. He covers them in Prior Analytics. So the general theory of dialectic deduction is covered, but particular methods of argument are not covered. Aristotle is only intending to teach about dialectical deduction itself. Learning how to construct proper deductions is something that Aristotle will teach somewhere else. It is for this reason that our understanding of dialectic will be adequate instead of complete.
Next, Aristotle will divide propositions into various groups.