What distinctions does Quine draw between ontological and ideological commitments?

Question

Quine held in "On What There Is" the following:

"A theory is committed to those and only those entities to which the bound variables of the theory must be capable of referring in order that the affirmations made in the theory be true."

and elsewhere...

"[…] this is essentially, the only way we can involve ourselves in ontological commitments; by bound variables."

It is thus by quantification that Quine considers a proposition to be bound to the existence of certain entities. This addresses his understanding of ontological commitment. But what does Quine have to say about ideological commitments (indeed, what does he consider 'ideological commitments' to be)? What difference divides such commitments from ontological commitments, and is it possible to have such commitments if one is not committing oneself to the existence, or ontological status, of whatever he is committing to ideologically?

Answer 1

Ontology and Ideology
Author(s): W. V. Quine
Source: Philosophical Studies: An International Journal for Philosophy in the Analytic Tradition
Location: Vol. 2, No. 1 (Jan., 1951), pp. 11-15
Stable URL: http://www.jstor.org/stable/431810

http://www.jstor.org/stable/431810

This being Quine we have the typical distortion and abuse of normal language. For that reason, as ultimately pointless though it is, we must first try to understand what Quine means when he uses the phrases 'ontological commitments' and 'ideological commitments'.

First of all Quine is speaking about the ontology of a theory and ideology of a theory. Or as he says:

ontology and ideology in their relativized aspect

We get a definition of ideology of a theory:

I have described the ideology of a theory vaguely as asking what ideas are expressible in the language of the theory.

He doesn't come out and say it straight but we can guess at his definition for ontology of a theory, presumably something along the lines, the ontology of a theory asks what can be said to exist as expressed in the language of the theory.

With those definitions (which already draw a distinction for us) let us turn to the explanations so we can further make clear the distinction, at least from Quine's perspective.

-- the ontology of a theory, the ideology of a theory-belong to what is commonly called semantics. But, as I have urged elsewhere, a fundamental cleavage needs to be observed between two parts of so-called semantics: the theory of reference and the theory of meaning. The theory of reference treats of naming, denotation, extension, coextensiveness, values of variables, truth; the theory of meaning treats of synonymy, analyticity, syntheticity, entailment, intension. Now the question of the ontology of a theory is a question purely of the theory of reference. The question of the ideology of a theory, on the other hand, obviously tends to fall within the theory of meaning;

So there you have it. Though now you do don't you wish you didn't? If we take meaning to be synonymous with sense then we have the old sense/reference dichotomy. To answer your question then, for Quine the difference between an ontological commitment and an ideological commitment is a distinction of reference versus sense.

Answer 2

See the following paper of Quine, I add a short clip.

Answer 3

The way I think about it is as follows: the ontology of a theory is given by the variables we quantify over to express the theory. The ideology is given by the semantics: what we take the predicates to be referring to (hence, not just truth-conditions).