Full Idea
The substratum [hupokeimenon?] is the material cause of the continuous occurrence of coming-to-be, because it is such as to change from contrary to contrary.
Gist of Idea
The substratum changing to a contrary is the material cause of coming-to-be
Source
Aristotle (Coming-to-be and Passing-away (Gen/Corr) [c.335 BCE], 319a19)
Book Reference
Aristotle: 'The Basic Works of Aristotle', ed/tr. McKeon,Richard [Modern Library Classics 2001], p.483
A Reaction
Presumably Aristotle will also be seeking the 'formal' cause as well as the 'material' cause (not to mention the 'efficient' and 'final' causes).