This essay is the first part of an analysis on the form and matter in the works of Aristotle. Within the whole analysis, I shall examine passages taken from different works of Aristotle that are relevant to the investigation on form and matter. In this essay, I shall focus exclusively on the chapter Metaphysics Zeta 3. The concepts of substance, matter, ontological subject, form, composite substance, this something and separated, which are consistently used by Aristotle within the development of the (...) mentioned chapter, will be part of my survey of the contents of the chapter. The central argument of Metaphysics Zeta 3, which maintains the equivalence between substance and the feature represented by ontological subject, and which leads, through this first equivalence and through the equivalence between matter and ontological subject, to the result that matter is (the only) substance, will be investigated step by step so that all presuppositions, entailments, and consequences of the argument itself can be clearly shown. The problems which are caused by the mentioned equivalence of substance and ontological subject will be pointed out during my look at the chapter’s contents. In particular, Aristotle cannot accept that, in spite of the ontological subject being a correct feature of substance qua substance, substance is therewith reduced to matter. Being the ontological subject does not represent the only ontological feature of substance qua substance; the ontological features too of substance qua substance that are represented by being a this something and being separated belong to the concept of substance: they can never be forgotten within a right interpretation of substance. The identification of substance only with matter can never be accepted. Only the ontological values of substance represented by the form and by the composite substance possess the ontological features “being a this something” and “being separated”. Therefore, the ontological values of substance as form and as a composite substance must always be reckoned with in a correctly interpreted ontology. Neither of these values can be forgotten within an accurate inquiry of ontology. Hence, regardless of whether matter is substance and is correctly interpreted as substance, the values of substance as form and as composite substance must belong, in Aristotle’s view, to any right ontological system whatsoever. The plurality of values for substance, which I personally advocate, finds confirmation thanks to chapter Metaphysics Zeta 3: Matter, form, and composite substance all represent values for substance, despite the differences which they have as regards their own ontological features. (shrink)