[4] She developed the first formal systems of quantified modal logic[5] and in so doing introduced the schema or principle known as the Barcan formula.

)[4] Marcus, who originally published as Ruth C. Barcan,[5] was, as Don Garrett notes[2] "one of the twentieth century's most important and influential philosopher-logicians".

Timothy Williamson, in a 2008 celebration of Marcus' long career, states that many of her "main ideas are not just original, and clever, and beautiful, and fascinating, and influential, and way ahead of their time, but actually – I believe – true".

[6] Ruth Barcan (as she was known before marrying the physicist Jules Alexander Marcus in 1942[7]) graduated magna cum laude from New York University in 1941, majoring in mathematics and philosophy.

From 1964 to 1970, she was a professor of philosophy at the University of Illinois Chicago (originally serving as a head of department).

Lewis gives Marcus special recognition in his "Notes on the Logic of Intension", originally printed in Structure, Method, and Meaning: Essays in Honor of Henry M. Sheffer (New York, 1951).

[11] However, in a recent laudatio to Ruth Barcan Marcus, Professor Timothy Williamson says: One of the ideas in them that resonates most with current philosophy of language is that of proper names as mere tags, without descriptive content.

Rather, it is the idea, later developed by David Kaplan and others, that proper names are directly referential, in the sense that they contribute only their bearer to the propositions expressed by sentences in which they occur.

[12]The philosopher of language Stephen Neale has also argued against Professor Smith's claim in the Times Literary Supplement.

Her argument counts against a widely received view that systems of moral rules are inevitably inconsistent.

[14] It is proposed that believing is a relationship of an agent to a possible state of affairs under specified internal and external circumstances.