Some realists claim that theoretical entities like numbers and electrons are indispensable
for describing the empirical world. Motivated by the meta-ontology of
Quine, I take this claim to imply that, for some first-order theory 𝑇 and formula
𝛿(𝑥) such that 𝑇 ⊢ ∃𝑥𝛿 ∧ ∃𝑥¬𝛿, there is no first-order theory 𝑇′ such that (a) 𝑇
and 𝑇′ describe the 𝛿:s in the same way, (b) 𝑇′ ⊢ ∀𝑥𝛿, and (c) 𝑇′ is at least as
attractive as 𝑇 in terms of other theoretical virtues. In an attempt to refute the
realist claim, I try to solve the general problem of nominalizing 𝑇 (with respect
to 𝛿), namely to find a theory 𝑇′ satisfying conditions (a)–(c) under various precisifications thereof.

In particular, I note that condition (a) can be understood either in terms of syntactic or semantic equivalence, where the latter is strictly stronger
than the former. The results are somewhat mixed. On the positive side, even under
the stronger precisification of (a), I establish that (1) if the vocabulary of 𝑇 is
finite, a nominalizing theory can always be found that is recursive if 𝑇 is, and (2)
if 𝑇 postulates infinitely many 𝛿:s, a nominalizing theory can always be found that
is no more computationally complex than 𝑇. On the negative side, even under the
weaker precisification of (a), I establish that (3) certain finite theories cannot be
nominalized by a finite theory.