Aristotle has just finished explaining how similarities can help us construct definitions. He is now finished his discussion of the various tools of argument. He concludes his discussion of Book 1 of the Topics by explaining what he has done so far and what he is going to do next.
The instruments, then, whereby deductions are effected, are these- the commonplace rules, for the observance of which the aforesaid instruments are useful, are as follows.
The tools of argument have now been discussed. Next we have to discuss the common rules for how to use these tools to construct arguments.
Remember that Aristotle has discussed a number of topics in Book 1. First, he discussed the subject of dialectic in general. He explained what it was, why we learn about it and how we will know when we have learned it. Next, he divided the topics that could be discussed into four and explained what they were. He also explained how these categories related to identity and his categories. Finally, he started discussing how we would gain the right materials to argue with. He discussed dialectic principles and definitions in general and then discussed the four tools of dialectic.
He needs to discuss how to use these tools before going on to discuss how to argue particular kinds of subjects. Both of these things will come later.
Aristotle started this particular chapter of the Topics, Book 1 by saying that he would say what deduction is and what the various kinds of deduction are. The last argument claimed that even specialized arguments were not exceptions to his division. Aristotle continues,
The foregoing must stand for an outline survey of the species of deduction. In general, in regard both to all that we have already discussed and to those which we shall discuss later, we may remark that that amount of distinction between them may serve, because it is not our purpose to give a precise definition of any of them- we merely want to describe them in outline: we consider it quite enough from the point of view of the line of inquiry before us to be able to recognize each of them in some sort of way.
What Aristotle previously argued for is only enough to distinguish between the various forms of deduction. We do not need to be more precise than that because we should focus on the objective of this particular book. This book only requires that we recognize dialectic deduction since that is what it discusses.
Aristotle has correctly described his position. There is no obvious reason why deduction cannot be divided the way that he suggests. He does not need an argument for the division. He only needs enough to recognize the division and for the division to be true. Arguments that a proper categorization of deduction is dialectic deduction and demonstrative deduction will have to come from somewhere else.
Having completed this task, we must continue on to the reasons for studying dialectic.