Full Idea
We come to have the ideas of particular sorts of substance, by collecting such combinations of simple ideas as are by observation of men's senses taken notice of to exist together, and are supposed to flow from the unknown essence of that substance.
Gist of Idea
We identify substances by supposing that groups of sensations arise from an essence
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.23.03)
Book Reference
Lowe,E.J.: 'Locke on Human Understanding' [Routledge 2004], p.296
A Reaction
Locke is notoriously somewhat ambiguous and unclear about some of his views, but this remark seems to make him the father of modern scientific essentialism. Note that this is an empiricist happily referring to an unperceived best explanation.