Full Idea
The only thing which is not predicated of some underlying thing is substance, while everything is predicated of it. But the same goes for substances too: there is something underlying them too, which they come from. Plants from seeds, for example.
Gist of Idea
Substance is not predicated of anything - but it still has something underlying it, that originates it
Source
Aristotle (Physics [c.337 BCE], 190b01)
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.26
A Reaction
[compressed] I presume 'substance' here is 'ousia'. Aristotle's quest is to pin down 'that which lies under', but this shows that if he identified it, he wouldn't have located what is ultimate. The explanation of a plant extends beyond the plant.
Related Idea
Idea 16173 Coming to be is by shape-change, addition, subtraction, composition or alteration [Aristotle]