Online research in philosophy

The thesis that the necessary and the a priori are extensionally equivalent consists of two independent claims: 1) All a priori truths are necessary and 2) all necessary truths are a priori. In Naming and Necessity1 Saul A. Kripke gives examples of necessary but a posteriori truths, so he disagrees with the second leg of the thesis.2 His examples are of two types; on the one hand statements involving essential properties and on the other hand true identity statements. My concern will be with examples of the second type and whether they refute (2). (2), however, is ambiguous and can mean one of three things.

The paper argues that, although a distinction between a priori and a posteriori knowledge (or justification) can be drawn, it is a superficial one, of little theoretical significance. The point is not that the distinction has borderline cases, for virtually all useful distinctions have such cases. Rather, it is argued by means of an example, the differences even between a clear case of a priori knowledge and a clear case of a posteriori knowledge may be superficial ones. In both cases, experience plays a role that is more than purely enabling but less than strictly evidential. It is also argued that the cases at issue are not special, but typical of a wide range of others, including knowledge of axioms of set theory and of elementary logical truths. Attempts by Quine and others to make all knowledge a posteriori (‘empirical’) are repudiated. The paper ends with a call for a new framework to be developed for analysing the epistemology of cognitive uses of the imagination.

Two-dimensional semantics aims to eliminate the puzzle of necessary a posteriori and contingent a priori truths. Recently many argue that even assuming two-dimensional semantics we are left with the puzzle of necessary and a posteriori propositions. Stephen Yablo (Pacific Philosophical Quarterly, 81, 98–122, 2000) and Penelope Mackie (Analysis, 62(3), 225–236, 2002) argue that a plausible sense of “knowing which” lets us know the object of such a proposition, and yet its necessity is “hidden” and thus a posteriori. This paper answers this objection; I argue that given two-dimensional semantics you cannot know a necessary proposition without knowing that it is true.

Suppose P is the conjunction of all truths statable in the austere vocabulary of an ideal physics. Then phsicalists are likely to accept that any truths not included in P are different ways of talking about the reality specified by P. This ‘redescription thesis’ can be made clearer by means of the ‘strict implication thesis’, according to which inconsistency or incoherence are involved in denying the implication from P to interesting truths not included in it, such as truths about phenomenal consciousness. Commitment to the strict implication thesis cannot be escaped by appeal to a posteriori necessary identities or entailments. A minimal physicalism formulated in terms of strict implication is preferable to one based on a priori entailment.

Chalmers argues that zombies are possible and that therefore consciousness does not supervene on physical facts, which shows the falsity of materialism. The crucial step in this argument – that zombies are possible – follows from their conceivability and hence depends on assuming that conceivability implies possibility. But while Chalmers’s defense of this assumption – call it the conceivability principle – is the key part of his argument, it has not been well understood. As I see it, Chalmers’s defense of the conceivability principle comes in his response to the so-called objection from a posteriori necessity. The defense aims at showing that there is no gap between conceivability and possibility since no such gap can be generated by necessary a posteriori truths. I will argue that while Chalmers is right to the extent that there is no gap between conceivability and possibility within the standard Kripkean model of a posteriori necessity, his general conclusion is not justified. This is because the conceivability principle might be inconsistent with a posteriori necessity understood in some non-Kripkean way and Chalmers has not shown that no such alternative understanding of a posteriori necessity is available.

My target in this paper is a view that has sometimes been called the ‘Linguistic Doctrine of Necessary Truth’ (L-DONT) and sometimes ‘Conventionalism about Necessity’. It is the view that necessity is grounded in the meanings of our expressions—meanings which are sometimes identified with the conventions governing those expressions—and that our knowledge of that necessity is based on our knowledge of those meanings or conventions. In its simplest form the view states that a truth, if it is necessary, is necessary (and knowably necessary) because it is analytic. It is widely recognized that this simple version of the view faces a prima facie problem with the existence of the necessary a posteriori. Assuming that all analytic truths are a priori, if there are necessary a posteriori truths then there are necessary synthetic truths—contradicting the view’s claim that all necessary truths are analytic. Contemporary L-DONTers have things to say about the problem, but in this paper I want to suggest that there is a different, more serious, problem which arises from the phenomenon of indexicality, which L-DONTers have not taken account of. Though there are many versions of the problem, a simple one is this. Consider Kaplan’s celebrated sentence.

The classical view of the relationship between necessity and apriority, defended by Leibniz and Kant, is that all necessary truths are known a priori. The classical view is now almost universally rejected, ever since Saul Kripke and Hilary Putnam discovered that there are necessary truths that are known only a posteriori. However, in recent years a new debate has emerged over the epistemology of these necessary a posteriori truths. According to one view – call it the neo-classical view – knowledge of a necessary truth always depends on at least one item of a priori knowledge. According to the rival view – call it the neoempiricist view – our knowledge of necessity is sometimes broadly empirical. In this paper I present and defend an argument against the neo-empiricist view. I argue that knowledge of the necessity of a necessary truth could not be broadly empirical.

In a series of influential articles, George Bealer argues for the autonomy of philosophical knowledge on the basis that philosophically known truths must be necessary truths. The main point of his argument is that the truths investigated by the sciences are contingent truths to be discovered a posteriori by observation, while the truths of philosophy are necessary truths to be discovered a priori by intuition. The project of assimilating philosophy to the sciences is supposed to be rendered illegitimate by the more or less sharp distinction in these characteristic methods and its modal basis. In this article Bealer's particular way of drawing the distinction between philosophy and science is challenged in a novel manner, and thereby philosophical naturalism is further defended.

We think that Kripke’s arguments that there are contingent a priori truths and that there are necessary a posteriori truths about named and essentially described entities fail. They fail for the reasons that there are ambiguities in each of the three eases. In the first ease, what is known apriori is not what is contingent. In the latter two cases, what is necessary or essential is not what is known a posteriori.