Abstract: This book is a collection of papers by Ruth Barcan Marcus, covering much ground in the development of her thought, and spanning from 1961 to 1990. Many of the papers deal with logical, semantic, metaphysical, and epistemological issues in intensional logic, and in particular, modalities. Some important themes that run through these papers are extensionality, the necessity of identity, the directly referential conception of proper names as “tags,” essentialism, substitutional quantification, and possibilia and possible worlds. What emerges from them is a robust defense of quantified modal logic in the light of a host of objections, particularly from Quine. Modalities also includes two papers on belief, which have consequences for epistemic logic and more widely for theories of rationality; two papers on ethical issues, which have consequences for deontic logic and practical reasoning; and finally, two papers on historical figures, Spinoza and Russell, dealing with the ontological proof of God's existence, and the nature of particularity, identity, and individuation, respectively.

Keywords: belief, deontic logic, essentialism, Ruth Barcan Marcus, modal logic, moral dilemmas, philosophy of logic, proper names, quantified modal logic, Quine, rationality, Russell, Spinoza, substitutional quantification, tags