Aristotle examines the concepts of substance (ousia) and essence (to ti ên einai, "the what it was to be") in his Metaphysics (Book VII), and he concludes that a particular substance is a combination of both matter and form, a philosophical theory called hylomorphism.
ˈæɹɪstˌɑːɾəl ɛɡzˈæmɪnz ðə kˈɑːnsɛpts ʌv sˈʌbstəns ˈaʊʒə ænd ˈɛsəns tə tˈaɪ ˈɛɹn ˈaɪnaɪ, "ðə wʌt ɪt wʌz tə bˈiː"ɪn hɪz mˌɛɾəfˈɪzɪks bˈʊk ɹˌoʊmən sˈɛvən, ænd hiː kəŋklˈuːdz ðˌæɾə pɚtˈɪkjʊlɚ sˈʌbstəns ɪz ɐ kˌɑːmbᵻnˈeɪʃən ʌv bˈoʊθ mˈæɾɚ ænd fˈɔːɹm, ɐ fˌɪləsˈɑːfɪkəl θˈiəɹi kˈɔːld hˈaɪləmˌoːɹfɪzəm.