Full Idea
I have defined the concept of power by saying that A exercises power over B when A affects B in a manner contrary to B's interests
Gist of Idea
Power is affecting a person in a way contrary to their interests
Source
Steven Lukes (Power: a Radical View (2nd ed) [2005], 1.6)
Book Reference
Lukes,Steven: 'Power: a radical view' [Macmillan 2005], p.37
A Reaction
I assume he is not referring to when I accidentally spill your beer. His point is, I think, that neither A nor B may be fully, or even partly, aware of what is going on. Presumably A can also exert power over B which is in B's interests. Dentists.