Full Idea
No one supposes a single 'element' to persist, as the basis of all, in such a way that it is Water as well as Air (or any other element) at the same time.
Gist of Idea
There couldn't be just one element, which was both water and air at the same time
Source
Aristotle (Coming-to-be and Passing-away (Gen/Corr) [c.335 BCE], 332a09)
Book Reference
Aristotle: 'The Basic Works of Aristotle', ed/tr. McKeon,Richard [Modern Library Classics 2001], p.515
A Reaction
Of course, we now think that oxygen is a key part of both water and of air, but Aristotle's basic argument still seems right. How could multiplicity be explained by a simply unity? The One is cool, but explains nothing.