Quasi-Implicative Lattices and the Logic of Quantum Mechanics

Mittelstaedt has defined the class of quasi-implicative lattices and shown that an ortholattice (orthocomplemented lattice) is quasi-implicative exactly if it is orthomodular (quasi-modular). He has also shown that the quasi-implication operation is uniquely determined by the quasi-implicative conditions. One of Mittelstaedt's conditions, however, seems to lack immediate intuitive motivation. Consequently, this paper seeks to provide a number of reformulations of the quasi-implicative conditions which are more intuitively plausible. Three sets of conditions are examined, and it is shown that each set of conditions is both necessary and sufficient to ensure that an ortholattice is orthomodular, and each set of conditions uniquely specifies the implication operation to be Mittelstaedt's quasi-implication. Various properties of the quasi-implication are then investigated. In particular, it is shown that the quasi-implication fails to satisfy a number of laws associated with the classical material conditional. Various weakenings of these laws, satisfied by the quasi-implication, are also discussed

Decomposition of Projections on Orthomodular Lattices(1)

The set of projections in the BAER*-semigroup of hemimorphisms on an orthomodular latticeLcan be partially ordered such that the subset of closed projections becomes an orthocomplemented lattice isomorphic to the underlying latticeL. The set of closed projections is identical with the set of Sasaki-projections onL(Foulis [2]). Another interesting class of (in general nonclosed) projections, first investigated by Janowitz [4], are the symmetric closure operators. They map onto orthomodular sublattices where Sasaki-projections map onto segments of the latticeL.

Modal propositional logic on an orthomodular basis. I

A common method of obtaining the classical modal logics, for example the Feys system T, the Lewis systems, the Brouwerian system etc., is to build on a basis for the propositional calculus by adjoining a new symbol L, specifying new axioms involving L and the symbols in the basis for PC, and imposing one or more additional transformation rules. In the jargon of algebraic logic, which is the point of view we shall adopt, the “necessity” symbol L may be interpreted as an operator on the Boolean algebra of propositions of PC. For example, the Lewis system S4 may be regarded as a Boolean algebra ℒ together with an operator L on ℒ having the properties: (1)Lp ≤ p for all p in ℒ, (2)L1 = 1, (3) L(p → q) ≤ Lp → Lq for all p, q in ℒ, and (4) Lp = L(Lp) for all p in ℒ. Here, of course, → denotes the material implication connective: p→q = p′ ∨ q. It is easy to verify that property (3) may be replaced by either (3′) L(p ∧ q) = Lp ∧ Lq for all p, q in ℒ, or by (3″) L(p → q) ∧ Lp ≤ Lq for all p, q in ℒ. In particular, it follows from (1) through (4) above that L is a decreasing, idempotent and isotone operator on ℒ. Such mappings are often called interior operators.In a previous paper [5], we considered the problem of introducing an implication connective into a quantum logic. This is greatly complicated by the fact that the quantal propositions band together to form an orthocomplemented lattice which is only “locally” distributive. Such lattices are called orthomodular. For definitions and further discussion, the reader is referred to that paper. In it, we argued that the Sasaki implication connective ⊃ defined by p ⊃ q = p′ ∨ (p ∧ q) is a natural generalization of material implication when the lattice of propositions is ortho-modular. Indeed, if unrestricted distributivity were permitted, p ⊃ q would reduce to the classical material implication p → q. For this reason, we choose ⊃ to play the role of material implication in an orthomodular lattice. Further properties of ⊃ are enumerated in Example 2.2(1) and Corollary 2.4 below.

On M-Symmetric Lattices

Every ⊥-symmetric relatively semi-orthocomplemented lattice is M-symmetric. This answers the Problem 1 in [2] in the affirmative and provides a new proof to a result on ⊥-symmetric lattices proved in [2] (Corollary below). The notation and terminology are as in [2].Let 〈L; ⋀, V〉 be a lattice. Two elements a and b of L are said to form a modular pair, in symbols aMb, ifThe relation aM✶b is defined dually.