Abstract
Paul Thagard’s theory of deductive coherence, as set out in his Coherence in Thought and Action, faces a regress objection. Thagard’s method of solving deductive coherence problems presupposes some notion of logical consequence. The problem of specifying which logic to use to this end is itself a deductive coherence problem, so we would expect Thagard’s theory to be able to solve it. However, on pain of regress, the theory of deductive coherence cannot reach such a solution.
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Notes
Of the four applications discussed in detail by Thagard (coherence theories of knowledge, coherence theories of truth, coherence theories of ethics and politics, and coherence theories of emotion), deductive coherence plays a role in at least three (all but emotion). See Thagard (1998) for an application of the theory of deductive coherence in ethics.
I thank an anonymous referee for suggesting this example.
One might object that coherence is a matter of degree, and that it is sometimes inevitable to have some degree of coherence between random elements. But even so, the argument stands: if the use of a non-classical logic generally leads to a lesser degree of coherence between random elements, it should be preferred over \(\mathbf {CL}\).
Perhaps Thagard himself has in mind a relevance-based system, for in a discussion of his theory of explanatory coherence he writes that “explanation is a more restrictive relation than deductive implication, because otherwise we could prove that any two propositions cohere; for unless we use a relevant logic (Anderson and Belnap 1975), \(P_1\) and the contradiction \(P_2\) & \(not-P_2\) imply any \(Q\), so it would follow that \(P_1\) coheres with \(Q\)” (Thagard 1989 p. 437).
Here, a logical pluralist may argue that the solution of different coherence problems requires different formal systems. Such a move will not block our objection, but will turn it into an infinite regress argument. The pluralist may argue that we need a different system, say \(\mathbf {L'}\), for solving \(P'\). This in turn generates the new deductive coherence problem \(P''\) of determining the definition of \(\mathbf {L'}\). For solving \(P''\) we need a (possibly different) formal system \(\mathbf {L''}\) for plugging in to (D2). This generates the deductive coherence problem \(P'''\), the solution of which requires a (possibly different) formal system \(\mathbf {L'''}\) for plugging in to (D2), and so on, ad infinitum.
See Haack (1982) for a criticism of Dummett’s views on the justification of deductive inference.
References
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Acknowledgments
Research for this paper was conducted at the Instituto de Investigaciones Filosóficas (UNAM) and supported by subventions of the Programa de Becas Posdoctorales de la Coordinación de Humanidades (UNAM).
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Beirlaen, M. A Regress Objection to Thagard’s Theory of Deductive Coherence. Erkenn 80, 975–986 (2015). https://doi.org/10.1007/s10670-014-9692-z
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DOI: https://doi.org/10.1007/s10670-014-9692-z