In Book VIII, he distinguishes the matter of the substance as the substratum, or the stuff of which it is composed.
ɪn bˈʊk ɹˌoʊmən ˈeɪt, hiː dɪstˈɪŋɡwɪʃᵻz ðə mˈæɾɚɹ ʌvðə sˈʌbstəns æz ðə sʌbstɹˈɑːɾəm, ɔːɹ ðə stˈʌf ʌvwˈɪtʃ ɪɾ ɪz kəmpˈoʊzd.