  Locke takes the notion of impulse, in which bodies communicate motion to one another by surface impact, to be, along with extension and cohesion, fundamental to our concept of  body.[21] Indeed, regardless of how motion may in fact be communicated, impulse is the only means by which we can conceive of its being communicated, a view of our conceptual abilities that Locke maintains, as we saw, despite his changing thoughts about gravity. Impulse is also fundamental to the corpuscular hypothesis’ explanation of phenomena, being either the exclusive means of interaction among bodies, as adherents of the contact action proviso hold, or the means of at least many interactions. Yet how exactly does a moving body communicate motion to a resting one simply by impacting it? When we attempt to discover the precise nature of the process, Locke suggests, we find that it is just as mysterious as the process by which the mind moves the body.