Full Idea
If Mill has a demon who, every time two things are brought together with two other things, always introduces a fifth, then if two black marbles and two white ones are put in an urn, the demon could choose his color, but there would be more of one colour.
Gist of Idea
If two black and two white objects in practice produced five, what colour is the fifth one?
Source
comment on John Stuart Mill (System of Logic [1843]) by C.I. Lewis - A Pragmatic Conception of the A Priori p.367
Book Reference
Peirce,James,Dewey etc: 'Pragmatism - The Classic Writings', ed/tr. Thayer,H.S. [Hackett 1982], p.367
A Reaction
Nice to see philosophers fighting back against demons. This is a lovely argument against the absurdity of thinking that experience could ever controvert a priori knowledge (though Lewis is no great fan of the latter).