Full Idea
For Locke, a real essence is what makes something what it is, and in the case of physical substances, it is the underlying physical cause of the object's observable qualities.
Gist of Idea
Lockean real essence makes a thing what it is, and produces its observable qualities
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by Jan-Erik Jones - Real Essence Intro
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.1
A Reaction
A helpful summary from a Locke expert. Is 'what it is' its type, or its individuality? Is the 'underlying cause' sufficiently coherent, or is it just a tangle of unseen activities?
Related Idea
Idea 16031 Real essence explains observable qualities, but not what kind of thing it is [Locke, by Jones,J-E]