Given a orthocomplemented lattice Q, a Mackey observable φ is a countably additive homomorphism from the orthocomplemented lattice of Borel subsets of R to Q. In symbols, this means that for any sequence { S i } i of pairwise-disjoint Borel subsets of R, {φ(S i)} i are pairwise-orthogonal propositions (elements of Q) and