We read in the seventh book of the Aristotelian “Metaphysics”: “And indeed the question which, both now and of old, has always been raised, and always been the subject of doubt, viz. What being is, is just the question, what is substance?” (Z1.1028b3-4. Tr. W. Ross, rev. by J. Barnes). The article introduces first (I), this question. Second (II), it analyses the Aristotelian answer by an interpretation of Z17.1041b4-9. The Aristotelian substance is “the occurrence of an essence”. Third (III), it gives some general remarks. It concludes with a question that Plato could have asked his rebel "son".