Quine's criterion

Statement

Willard Van Orman Quine provided an early and influential formulation of ontological commitment: If one affirms a statement using a name or other singular term, or an initial phrase of 'existential quantification', like 'There are some so-and-sos', then one must either (1) admit that one is committed to the existence of things answering to the singular term or satisfying the descriptions, or (2) provide a 'paraphrase' of the statement that eschews singular terms and quantification over so-and sos. Quine's criterion can be seen as a logical development of the methods of Bertrand Russell and G.E. Moore, who assumed that one must accept the existence of entities corresponding to the singular terms used in statements one accepts, unless and until one finds systematic methods of paraphrase that eliminate these terms.

The purpose of Quine's strategy is to determine just how the ontological commitment of a theory is to be found. Quine argued that the only ontologically committing expressions are variables bound by a first-order existential quantifier, and natural language expressions which were formalized using variables bound by first-order existential quantifiers.

Attempts have been made to argue that predicates are also ontologically committing, and thus that subject-predicate sentences bear additional ontological commitment to abstract objects such as universals, sets, or classes. It has been suggested that the use of meaningful names in nonexistence statements such as "Pegasus does not exist" brings with it an ontological commitment to empty names like Pegasus, a quandary referred to as Plato's beard and escaped by using quantifiers.

This discussion has a connection to the Carnap–Quine argument over analytic and synthetic objects. Although Quine refers to 'ontological commitment' in this connection, in his rejection of the analytic/synthetic distinction he does not rely upon the formal translation of any particular theory along the lines he has suggested. Instead, Quine argues by using examples that although there are tautological statements in a formal theory, like "all squares are rectangles", a formal theory necessarily contains references to objects that are not tautological, but have external connections. That is, there is an ontological commitment to such external objects. In addition, the terms used to interpret the application of the theory are not simply descriptions of sensory input, but are statements in a context. That is, inversely, there is an ontological commitment of these observational objects to the formal theory. As Ryan puts it: "Rather than being divided between contingent synthetic claims and indubitable analytic propositions, our beliefs constitute a continuous range from a periphery of sense-reports to interior concepts that are comparatively theory-laden and general." Thus we end up with Quine's 'flat' ontology that does not see a distinction between analytic and synthetic objects.

Quine further made a distinction between the ontological commitments of a theory (what the theory says exists) and the ideological commitments of a theory (those concepts, logical or non-logical, that are expressible within the theory).

Whatever process one uses to determine the ontological commitments of a theory, that does not prescribe what ontological commitments one should have. Quine regarded this as a matter of epistemology, which theory one should accept. "Appeal is made to [concerns of] explanatory power, parsimony, conservatism, precision, and so on".

Ontological parsimony can be defined in various ways, and often is equated to versions of Occam's razor, a "rule of thumb, which obliges us to favor theories or hypotheses that make the fewest unwarranted, or ad hoc, assumptions about the data from which they are derived." Glock regards 'ontological parsimony' as one of the 'five main points' of Quine's conception of ontology.

Following Quine, Baker states that a theory, T, is ontologically committed to items F if and only if T entails that *F′*s exist. If two theories, T₁ and T₂, have the same ontological commitments except that T₂ is ontologically committed to *F′*s while T₁ is not, then T₁ is more parsimonious than T₂. More generally, a sufficient condition for T₁ being more parsimonious than T₂ is for the ontological commitments of T₁ to be a proper subset of those of T₂.

These ideas lead to the following particular formulation of Occam's razor: 'Other things being equal, if T₁ is more ontologically parsimonious than T₂ then it is rational to prefer T₁ to T₂.' While a common formulation stipulates only that entities should not be multiplied beyond necessity, this version 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.