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Published online by Cambridge University Press: 01 January 2020
Alvin Plantinga has recently argued that there are certain propositions which are necessary but known only a posteriori. If Plantinga is correct then he has shown that the traditional view that all necessary truths are knowable a priori is false. Plantinga's examples deserve special attention because they differ in important respects from other proposed examples of necessary a posteriori truths. His examples depend on a certain conception of possible worlds and in particular on his conception of the actual world. It will be argued that these examples of necessary a posteriori propositions can be understood in two different ways. According to one way of understanding Plantinga, the propositions turn out to be contingent a posteriori truths, and according to the other way they turn out to be necessary a priori truths. The plausibility of Plantinga's position is due to a confusion between the two possible interpretations.
I would like to thank Richard Foley for his helpful discussions with me on this topic.
2 Plantinga, Alvin The Nature of Necessity (Clarendon Press, 1974).Google Scholar
3 See Kripke, Saul A. “Naming and Necessity,” in Davidson, D. and Harman, G. eds., Semantics of Natural Languages (D. Reidel Publishing Company, 1972).Google Scholar Elsewhere I argue that Kripke's examples will not work, see my paper, “Are There Necessary A Posteriori Truths?” (Philosophical Studies, in press).
4 Op cit., p. 83.
5 It is important to note that (1) is not equivalent to (1’): Socrates is snubnosed in ɑ. (1’) is not a necessary truth according to Plantinga, since (1’) is true only in those worlds in which Socrates exists.
6 Plantinga, op. cit., p. 45.
7 Ibid., p. 47.
8 Op. cit., pp. 49–51.
9 For an explanation of ‘rigid designator’ see Kripke, op. cit.
10 Besides Plantinga's and Kripke's examples other cases of necessary a posteriori truths have been proposed; e.g., it has been suggested that there are computations so complex that they can be carried out only by a computer, and thus are necessarily true but known only a posteriori. It is true that given the current life span of humans there are computations that no human does carry out. However, the fact that humans currently live only a certain amount of time does not seem relevant to whether they could in principle carry out computations of this sort. It is only this latter consideration which is relevant to the a priori- a posteriori distinction. (See my paper “Are There Contingent A Priori Truths?”.)