Ruth Barcan Marcus, though she has published in a number of areas, is best known for her groundbreaking papers in modal and philosophical logic. In 1946 she initiated the first systematic treatment of quantified modal logic (see Barcan, 1946), therein provoking W. V. Quine's decades-long attack upon the meaningfulness of quantification into alethic modal contexts. The ensuing dispute focused attention on the phenomenon of referential opacity and led to important developments in logic, metaphysics, and philosophy of language. In subsequent papers Marcus extended the first-order formalization to second order with identity (Barcan, 1947) and to modalized set theory (Marcus, 1963, 1974). Particularly significant theses presented in these works were the axiom ♦(∃x)Fx → (∃x)♦Fx, known as the Barcan formula (Barcan, 1946), and the proof of the necessity of identity (Barcan, 1947; Marcus, 1961). It is of some historical interest that Marcus introduced the now standard "box" operator for necessity.

Marcus's response...