Third, Aristotle distinguishes between practical and theoretical knowledge in terms of the level of precision that can be attained when studying them. Political and moral knowledge does not have the same degree of precision or certainty as mathematics. Aristotle says at Ethics 1094b14: "Problems of what is noble and just, which politics examines, present so much variety and irregularity that some people believe that they exist only by convention and not by nature….Therefore, in a discussion of such subjects, which has to start with a basis of this kind, we must be satisfied to indicate the truth with a rough and general sketch: when the subject and the basis of a discussion consist of matters that hold good only as a general rule, but not always, the conclusions reached must be of the same order." Aristotle does not believe that the noble and the just exist only by convention, any more than, say, the principles of geometry do. However, the principles of geometry are fixed and unchanging. The definition of a point, or a line, or a plane, can be given precisely, and once this definition is known, it is fixed and unchanging for everyone. However, the definition of something like justice can only be known generally; there is no fixed and unchanging definition that will always be correct. This means that unlike philosophers such as Hobbes and Kant, Aristotle does not and in fact cannot give us a fixed set of rules to be followed when ethical and political decisions must be made. Instead he tries to make his students the kind of men who, when confronted with any particular ethical or political decision, will know the correct thing to do, will understand why it is the correct choice, and will choose to do it for that reason. Such a man will know the general rules to be followed, but will also know when and why to deviate from those rules. (I will use "man" and "men" when referring to citizens so that the reader keeps in mind that Aristotle, and the Greeks generally, excluded women from political part icipation. In fact it is not until the mid-19th century that organized attempts to gain the right to vote for women really get underway, and even today in the 21st century there are still many countries which deny women the right to vote or participate in political life).
For Aristotle, the form is not something outside the object, but rather in the varied phenomena of sense. Real substance, or true being, is not the abstract form, but rather the concrete individual thing. Unfortunately, Aristotle's theory of substance is not altogether consistent with itself. In the Categories the notion of substance tends to be nominalistic (that is, substance is a concept we apply to things). In the Metaphysics , though, it frequently inclines towards realism (that is, substance has a real existence in itself). We are also struck by the apparent contradiction in his claims that science deals with universal concepts, and substance is declared to be an individual. In any case, substance is for him a merging of matter into form. The term "matter" is used by Aristotle in four overlapping senses. First , it is the underlying structure of changes, particularly changes of growth and of decay. Secondly , it is the potential which has implicitly the capacity to develop into reality. Thirdly , it is a kind of stuff without specific qualities and so is indeterminate and contingent. Fourthly , it is identical with form when it takes on a form in its actualized and final phase.