Pluralism in Logic

A number of people have proposed that we should be pluralists about logic, but there are several things this can mean. Are there versions of logical pluralism that are both high on the interest scale and also true? After discussing some forms of pluralism that seem either insufficiently interesting or quite unlikely to be true, the paper suggests a new form which might be both interesting and true; however, the scope of the pluralism that it allows logic is extremely narrow. There are quite a few theses about logic that are in one way or another pluralist: they hold (i) that there is no uniquely correct logic, and (ii) that because of this, some or all debates about logic are illusory, or need to be somehow reconceived as not straightforwardly factual. Pluralist theses differ markedly over the reasons offered for there being no uniquely correct logic. Some such theses are more interesting than others, because they more radically affect how we are initially inclined to understand debates about logic. Can one find a pluralist thesis that is high on the interest scale, and also true? §1. The boundaries of logic. One form of pluralism that strikes me as true though of somewhat limited interest is Tarski’s (1936) thesis that there is no principled division of concepts into the logical and the nonlogical, and the related view that there is no principled division between logical truths and truths that don’t belong to logic.1 This seems plausible: there seems little point to a debate between a person who takes first-order logic with identity to be logic and someone who thinks that only first-order logic without identity is really logic. Well, there might be a point if the second person were to claim that some of the axioms of identity that the first person was proposing aren’t true, but I’m imagining that the two parties agree on the truth of the axioms of identity, they just disagree as to whether they should count as part of logic. Even then, there could be a substantive issue behind the scenes, if they took being logical as associated with some higher epistemological status than is attainable in the nonlogical realm. But it’s hard to find any plausible thesis according to which logic has this higher epistemological status; and even if such a thesis were assumed, it would be clearer to put the debate as about whether the laws of identity have this alleged special epistemic status. Some debates about the demarcation between logic and nonlogic can seem more interesting, but I doubt that the demarcation itself ever really is. Consider, for instance, disputes over ontological commitment. Someone who wants to avoid ontological commitment to sets, but still be able to talk about finitude and infinitude, might hold that Quine’s insistence Received: December 15, 2008 1 The view can concede that there are principled necessary conditions or principled sufficient conditions, or both; just no principled dichotomy. c © 2009 Association for Symbolic Logic 342 doi:10.1017/S1755020309090182 PLURALISM IN LOGIC 343 on assessing ontological commitment with respect to first-order logic is unnecessarily restrictive: one can talk of finitude and infinitude without use of set theory if one expands first-order logic to include the quantifier ‘there are infinitely many’ (or more powerful devices from which it can be defined). But here the issue isn’t really about the scope of logic, it is whether to allow the quantifier ‘there are infinitely many’ as primitive. Taking it to be primitive doesn’t require a decision on whether it is a logical or nonlogical primitive, or on which truths governing it count as logical truths. The situation is rather similar in the case of disputes as to whether full impredicative higher order logic should count as logic; but here it is more complicated because there are a larger number of genuine issues that are likely to underlie the dispute, and they interrelate in complex ways. Among the relevant issues are: (i) the grammatical issue of whether quantifying into the predicate position is legitimate; (ii) the issue of how the range of the quantifiers would have to be understood (sets? classes, including proper classes? properties? “pluralities” in the second-order case, somehow extended if one allows still higher order logic?); (iii) the issue of whether it is intelligible to construe the quantifiers impredicatively in these cases. (For instance, does it make sense to talk of impredicative proper classes, or to take one’s plural quantifiers to be impredicative when the “plurality” in question is too big to be a set?) (iv) various issues about ontological commitments of second-order quantifiers: for example, is there a sense in which these aren’t real ontological commitments, or alternatively, in which the ontological commitments they make are already implicit in sentences without second-order quantifiers? (v) the issue of whether second-order quantification is determinate. These and other issues are genuine, but can be separated from the issue of whether higher order logic is “really logic”. Indeed, it is desirable to separate them, because one might give answers of the sort associated with the thesis that “second-order logic is logic” to some but not all of them. In general, then, I’m inclined to agree with any pluralism based on the arbitrariness of the demarcation between logic and nonlogic. That kind of pluralism doesn’t strike me as altogether exciting (though I know it has been denied), but I’ll leave that for the reader to judge. §2. Radical pluralism. A quite exciting form of pluralism would be the claim that alternative logics (or any meeting minimal conditions) never genuinely conflict. Carnap appears to have suggested this, with his “Principle of Tolerance”: . . . let any postulates and rules of inference be chosen arbitrarily; then this choice, whatever it may be, will determine what meaning is to be assigned to the fundamental logical symbols. By this method, also, the conflict between the divergent points of view on the problem of the foundations of mathematics disappears. . . . The standpoint we have suggested—we will call it the Principle of Tolerance . . . —relates not only to mathematics, but to all questions of logic. (Carnap, 1934, p. xv) In discussing this form of pluralism, it would be a mistake to engage in debates on such