but since they are free of any hierarchy of types, they have no need for a distinct universal class at each type level. Quine's set theory and its background logic were driven by a desire to minimize posits; each innovation is pushed as far as it can be pushed before further innovations are introduced. For Quine, there is but one connective, the Sheffer stroke, and one quantifier, the universal quantifier. All polyadic predicates can be reduced to one dyadic predicate, interpretable as set membership. His rules of proof were limited to modus ponens and substitution. He preferred conjunction to either disjunction or the conditional, because conjunction has the least semantic ambiguity. He was delighted to discover early in his career that all of first order logic and set theory could be grounded in a mere two primitive notions: abstraction and inclusion. For an elegant introduction to the parsimony of Quine's approach to logic, see his "New Foundations for Mathematical Logic", ch. 5 in his From a Logical Point of View. Metaphysics Quine has had numerous influences on contemporary metaphysics. He coined the term "abstract object". He also coined the term "Plato's beard" to refer to the problem of empty names. Rejection of the analytic–synthetic distinction In the 1930s and 40s, discussions with Rudolf Carnap, Nelson Goodman and Alfred Tarski, among others, led Quine to doubt the tenability of the distinction between "analytic" statements—those true simply by the meanings of their words, such as "No bachelor is married"— and "synthetic" statements, those true or false by virtue of facts about the world, such as "There is a cat on the mat." This distinction was central to logical positivism. Although Quine is not normally associated with verificationism, some philosophers believe the tenet is not incompatible with his general philosophy of language, citing his Harvard colleague B. F. Skinner and his analysis of language in Verbal Behavior. Like other analytic philosophers before him, Quine accepted the definition of "analytic" as "true in virtue of meaning alone". Unlike them, however, he concluded that ultimately the definition was circular. In other words, Quine accepted that analytic statements are those that are true by definition, then argued that the notion of truth by definition was unsatisfactory. Quine's chief objection to analyticity is with the notion of cognitive synonymy (sameness of meaning). He argues that analytical sentences are typically divided into two kinds; sentences that are clearly