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.