CDD: 501 TRUTHS ANCIENT AND MODERN1 DAVID MILLER Department of Philosophy University of Warwick Coventry CV4 7AL UK [email protected] Abstract: The paper presents a comparison of the theories of truth, and the sol- utions of the liar paradox, proposed by Thomas Bradwardine (c. 1290–1349), Jean Buridan (c. 1295–1358), and Alfred Tarski (1901–1983). Keywords: Self-reference. Bradwardine. Buridan. Tarski. Liar paradox. Method of diagonalization. Theory of signification. Theories of truth. 1. INTRODUCTION The subject of this paper is truth, and its opposite, falsity, es- pecially when they live together. That is to say, I wish to turn my attention once again to the ancient paradox of the liar, one of the most venerable problems of logic. My pretext for returning to a subject so familiar and so deeply explored by others is the recent revival by my 1This is an augmented and corrected version of an invited lecture given at the meeting Science, Truth and Consistency, held at the State Uni- versity of Campinas (UNICAMP) from 25–28 August 2009 in celebration of the 80th birthday of Newton da Costa. Versions of the lecture have been giv- en also at Kyoto University in July 2009, at the XVII Foro Nacional de Filosofía de Colombia, held at the University of Caldas (sede Palogrande) from 15–18 September 2009, at the University of Valparaíso in October 2009, and at the University of Edinburgh in March 2010. I am indebted to Stephen Read for several valuable comments and corrections. The Spanish version of the lecture is expected to appear in due course in the journal Discusiones Filosóficas (Manizales), together with a brief response from Read. Manuscrito — Rev. Int. Fil., Campinas, v. 34, n. 1, p. 267-292, jan.-jun. 2011. 268 DAVID MILLER compatriot, the philosopher Stephen Read, of a resolution of this para- dox, and others, that was proposed in the 14th century by Thomas Bradwardine, a little known Englishman today, but a man of much intellectual distinction. Read (2008a, 2008b, 2010) not only presents Bradwardine’s theory of truth sympathetically in modern dress. He is also an enthusiastic proponent of it, maintaining that it is in various respects superior to the predominant theory today; that is, the theory due to Alfred Tarski. I am by no means in agreement with this judge- ment. I think, nevertheless, that Bradwardine’s contribution deserves our attention, and that we can learn much from an investigation, even a brief one, into this new-old solution. 2. THE LIAR PARADOX In speaking of the liar paradox, I may take for granted the general idea, that is, of a sentence, to all appearances well constructed and in- telligible, that is true if & only if it is false (or not true). The paradox has assumed many different forms in the course of its history, from the most everyday to the most mathematical. There are simple statements such as ‘I am lying’, in which there is an implicit reference to the time at which the statement is made. There are additionally the sentences ‘This sentence states something false’ and ‘This is a false sentence’, which use demonstrative adjectives or pronouns to indicate what they are talking about. I shall follow here the common custom of calling such sentences self-referential, by which I mean that it is the gramma- tical subject of the sentence, though not the sentence itself, that refers to the whole sentence. An important version of the liar paradox uses only indirect self- reference, and shows that direct self-reference does not constitute the heart of the problem. If, for example, Daphnis says that Chloe is speaking falsely, and Chloe says that Daphnis is speaking truly, then Manuscrito — Rev. Int. Fil., Campinas, v. 34, n. 1, p. 267-292, jan.-jun. 2011. TRUTHS ANCIENT AND MODERN 269 there is no consistent attribution of truth values to these utterances. If Daphnis’ utterance is true, then Chloe’s is false, in which case Daph- nis’ utterance is also false; and hence Chloe’s utterance is true, and so Daphnis’ utterance is true after all. We have travelled in a complete circle without any stable attribution of truth or falsity to either of the two utterances. There are versions of the paradox that depend for their effectiveness on an empirical premise that does not appear explicitly. For example, there is the paradox of Epimenides the Cretan, who said that all Cre- tans are liars; that is, that all Cretans persistently and unfailingly utter falsehoods. This utterance of Epimenides cannot be true, which implies that not all Cretans unfailingly utter falsehoods. That is to say, there is some Cretan, Epimenides or someone else, who on one occasion at least utters something true. It is astonishing that Epimenides’ mere utterance of what he uttered can have such an outcome. The inverse of this paradox is Curry’s paradox (Goldstein (1986)), a sentence such as ‘If this sentence is true then God exists’. If this condi- tional sentence is false, its antecedent is true, and hence the conditional is not false but true, and, what is more, has a true antecedent. Using the rule of modus ponens (or the truth table for the conditional) we may conclude that the consequent is true also; that is, that God exists. In such a way we may demonstrate ab initio any sentence that we wish to demonstrate. I should mention also the so-called paradox of the truth-teller, a sen- tence that attributes truth, and nothing more, to itself. The problem here is that there seems to be no consideration whatever that could help us to assign a truth value to this sentence. In this paper, in order to avoid problems associated with the use of demonstrative pronouns and adjectives, indexical expressions, and other idiomatic locutions that introduce into the discussion elements that are far from formal and at times undeniably empirical, I shall Manuscrito — Rev. Int. Fil., Campinas, v. 34, n. 1, p. 267-292, jan.-jun. 2011. 270 DAVID MILLER mean by the liar paradox some version generated by the method of diagonalization invented by Gödel in 1931. What follows is an adaptation of a construction by Tarski that is hidden in note 11 of his popular article (1944), and rarely mentioned. If y is a sentence of the form ‘b has B’, let the transform y? of y be the sentence ‘ “b has B” has B’. For example, y may be the true sentence ‘The first sentence of the first published English translation of Witt- genstein’s Tractatus has exactly eight words’, in which case y? is the false sentence ‘ “The first sentence of the first published English trans- lation of Wittgenstein’s Tractatus has exactly eight words” has exactly eight words’. Let z be the sentence ‘b has a false transform’. Then z?, the transform of z, is ‘ “b has a false transform” has a false transform’. It is intuitively evident that z? is true if & only if the transform of z is false, that is, if & only if z? is false. We may construct likewise an example of the truth-teller paradox. Let x be the sentence ‘b has a true transform’. Its transform x?, that is, ‘ “b has a true transform” has a true transform’ is equivalent to the sentence ‘x? is true’. Henceforth z? (or any other similar sentences constructed by the method of diagonalization) will be given the name U, and called a liar. A sentence like x? will be called a truth teller. Note, however, that the grammatical subject of the sentence U, that is, the clausal name ‘“b has a false transform” ’ does not refer to U (but to z). U is not a self-referential sentence in the usual sense, though it says something about itself. We must note also a point of great importance: that if we assume the classical truth table for the operation of negation, that is, that a sentence y is true if & only if its negation ¬ y is false, then the nega- tion ¬ U of U is equally a liar, since ¬ U is true if & only if U is false, that is, if & only if U is true, that is, if & only if ¬ U is false. This sentence ¬ U, that is, ‘ “b has a false transform” has a true transform’, may seem to be even less self-referential than is U, since we cannot Manuscrito — Rev. Int. Fil., Campinas, v. 34, n. 1, p. 267-292, jan.-jun. 2011. TRUTHS ANCIENT AND MODERN 271 identify ¬ U with any expression mentioned in ¬ U. Each of Uand ¬ U says something about U, but only one of them actually is U. But this asymmetry is an illusion. To see this, let the countertransform y† of a sentence y of the form ‘b has B’ be the sentence ‘the sentence ‘ “b has B” does not have B’. Let z be the sentence ‘b has a false transform’, as before, and let z†, the countertransform of z, be called W . Then W is interdeducible with ¬ U, and ¬ W is interdeducible with U. Each of W and ¬ W says something about W , but only one of them actually is W . In short, ¬ U, which is equivalent to W , is just as self-referential as U is. We shall sometimes say that U, and other liars such as ¬ U, W , and ¬ W exhibit the phenomenon of reflexivity. An old solution to the liar paradox, formulated a century before Bradwardine, is that of cassationism (Goldstein (2008)), which judges that reflexive locutions are often, or always, senseless, saying nothing, and should be excluded from language.