 Although the word “prime” does not occur here, Aristotle is evidently talking about prime matter. A natural way to read this passage is that he is saying there is a wholly indeterminate underlying thing, which he calls “matter”, and it is not a substance. Those who wish to avoid attributing a doctrine of prime matter to Aristotle must offer a different interpretation: that if we were to make the mistake of regarding matter, as opposed to form, as substance, we would be committed (absurdly) to the existence of a wholly indeterminate underlying thing.