  For Russell, sense data are the direct objects of thought in terms of which we think about extra-mental reality under descriptions relating, typically causally, those bits of reality to the directly referred to sense data. (It is worth noting that the basic idea does not require Russell's theory of perception. For example, John Searle, in (1983), has a similar view according to which we think immediately about experiences, which are not analyzed in terms of the mind's being presented with sense data. We then think of extra-mental objects as the causes of such and such occurrent experiences.) For Russell, while there are thoughts about particulars that are wholly qualitative and wholly general, the paradigm form our thought about external reality takes is ultimately grounded in acquaintance, albeit acquaintance with sense data as opposed to extra-mental reality itself. This is in stark contrast to the canonical form such thought takes in Frege's system, in which there is no direct reference to any individuals, including sense data, and all thought is entirely indirect. (For further discussion, see the entries  Bertrand Russell,  knowledge by acquaintance vs. description, and  descriptions.)