 What does it mean to say that one theory is more ontologically parsimonious than another? The basic notion of ontological parsimony is quite straightforward, and is standardly cashed out in terms of Quine’s concept of ontological commitment. A theory, \(T\), is ontologically committed to \(F\)s if and only if \(T\) entails that \(F\)’s exist (Quine 1981, pp. 144–4). If two theories, \(T_1\) and \(T_2\), have the same ontological commitments except that \(T_2\) is ontologically committed to \(F\)s and \(T_1\) is not, then \(T_1\) is more parsimonious than \(T_2\). More generally, a sufficient condition for \(T_1\) being more parsimonious than \(T_2\) is for the ontological commitments of \(T_1\) to be a proper subset of those of \(T_2\). Note that OR1 is considerably weaker than the informal version of Occam’s Razor, OR, with which we started. OR stipulates only that entities should not be multiplied beyond necessity. OR1, by contrast, states that entities should not be multiplied other things being equal, and this is compatible with parsimony being a comparatively weak theoretical virtue.