> That doesn't make him an empiricist, i.e. one who "regards experience
> as the only source of knowledge" (OED).
In his book on Hume, Deleuze says this definition is nonsense. And I agree. Under this definition, everyone is an empiricist. Or no one is. It doesn't capture what empiricism really investigates, which is really relation between things and their movement, instead of the nature of things. If you accept that definition, Marx is certainly an empiricist.