We may define a 'mind' as a collection of events connected with each other by memory-chains backwards and forwards. We know about one such collection of events - namely, that constituting ourself - more intimately and directly than we know about anything else in the world. In regard to what happens to ourself, we know not only abstract logical structure, but also qualities ... This is the sort of thing that we cannot know where the physical world is concerned.
Source: My Philosophical Development, chap. ２,1959
More info.: https://russell-j.com/beginner/BR_MPD_02-120.HTM