 The Bell-Kochen-Specker theorem is a corollary of Gleason’s theorem (Gleason 1957), though Bell and Kochen-Specker obtain it directly, and not via Gleason’s theorem, whose proof is considerably more intricate. The question addressed by Gleason has to do with assignments of probabilities to closed subspaces of a Hilbert space (or, equivalently, to projection operators onto such subspaces), such that the probabilities assigned to orthogonal projections are additive. Gleason proved that, in a Hilbert space of dimension 3 or greater, any such assignment of probabilities can be represented by a density operator. The BKS theorem deals with the special case in which the assignments are confined to the values 1 or 0.