Full Idea
Power being the source from whence all action proceeds, the substances wherein these powers are, when they exert this power into act, are called 'causes'.
Gist of Idea
Causes are the substances which have the powers to produce action
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.22.11)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.294
A Reaction
This is causes as actual entities, rather than as conjunctions of events. Personally I find this view of Locke's very congenial, no matter how unfashionable it may be.