The search for relationships between logic and algebra goes back to
the investigations of Boole and his followers. Those investigations yielded
what we now call Boolean algebra. The close links between classical
logic and the theory of Boolean algebras has been known for a long
time. One of the turning points in the algebraic study of logic was the
introduction by Lindenbaum and Tarski of the method of treating
formulas, or equivalence classes of formulas as elements of an abstract
algebra. Another milestone was the treatment of formulas as algebraic
functions in certain algebras. This is a generalization of truth-tables in
classical logic and can be found in the formulation by Lukasiewicz and
Post of many-valued logics and in the metalogical study of those systems.