Full Idea
Observing in ourselves that we can at pleasure move several parts of our bodies, which were at rest; the effects also that natural bodies are able to produce in one another, occurring every moment to our senses, we both these ways get the idea of power.
Gist of Idea
We get the idea of power from our own actions, and the interaction of external bodies
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.07.08)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.131
A Reaction
This I take to be one of the most important concepts in our understanding of the world, a concept which died out in the eighteenth century, and has now reappeared in scientific essentialism.