A thought today from Bertrand Russell's book, Human Knowledge. Russell is here discussing the relationship of cause and effect. He considers the possibility that the relationship might be entirely subjective, just a name for the fact that A regularly precedes B in my experience . Whenever I see a dry match struck against a rock, I then see a flame, etc. He refers to this hypothesis as that of a "world consisting only of data," because it involves seeing causation (and other relations) as connections between one datum and another, not as facts about some external world whence come the data into my subjective consciousness. And of that idea, he says this: "As regards the irregularity of a world consisting only of data, this is an argument to which it is difficult to give precision. Roughly speaking, many sensations occur without any fixed antecedents in our own experience, and in a manner which suggests irresistibly that, if they have causes, these causes lie ...