  Goodman and Quine construct a theory of syntax for the set-theoretic language and a proof theory based on the Calculus of  Individuals (see  section 3.2 below)   supplemented with a token-concatenation theory. The tokens in question are concrete, particular inscriptions of the logical symbols, variable-letters, parentheses, and the “\(\in\)” (for set-membership) that are use to formulate the language of set theory. Primitive predicates are introduced to categorize the different primitive symbols: all concrete, particular “\(\in\)”-inscriptions, for instance, fall under the predicate “Ep”. Concrete complex formulae, e.g.,  “\(x \in y\)”, are concatenations of concrete primitive symbols—in our case the concatenation of “\(x\)” and “\(\in\)” and “\(y\)”. Bit by bit, Goodman and Quine define their way up to which concrete inscriptions count as correctly formed sentences of the language of set theory, and finally which concrete inscriptions count as proofs and theorems. Goodman and Quine argue that in this way the nominalist can explain the “remarkable agreement” of mathematicians mentioned above.