The Mind/Brain Identity Theory:
A Critical Appraisal

1. Introduction

Citation Information

Allan, Leslie 2016. The Mind/Brain Identity Theory: A Critical Appraisal, URL = <>.

human brain depicted as interlocking cogs

Identity theories of mind can be categorized as those theories that contingently identify mental things, processes, states and events with physical things, processes, states and events. These theories can either be materialist, idealist (depending on the direction of reduction)[1] or neutral monist. For reasons that I will not explore here, I consider idealism and neutral monism to be untenable as ontological theories.[2] My interest in this essay is to consider what I think are the outstanding difficulties faced by a materialist identity theory.

I shall consider the most plausible type of materialist identity theory so far proposed; that which identifies the mind with the brain or, more accurately, the central nervous system, or part thereof. For the sake of convenience, I shall henceforth sacrifice precision and speak simply of the 'brain'. I shall begin by considering one common objection to a materialist identity theory. The discussion of the solution to this objection will serve to outline the direction that I think that a plausible identity theory should take and open the way for a critical survey of outstanding problems. I shall not consider problems that I believe have had a satisfactory solution expressed in the literature on this topic.

I want to say at the outset that I do not think that there are any logically sufficient reasons for rejecting any version of the identity theory. This is because I do not think that there are any such reasons for rejecting any internally consistent explanatory theory, including ontologies. (Ontologies are explanatory in the sense that they are attempts to explain the apparent diversity and the apparent unity of phenomena.) According to the well-known Duhem–Quine underdetermination thesis, explanatory theories do not entail observation statements in isolation. It is only when a theory is coupled with auxiliary hypotheses about initial conditions and other intervening mechanisms that an observation statement is entailed. Any apparent 'counterexample' to a theory can be dealt with in an ad hoc fashion, with the result that the apparent 'counterexample' turns out either to be a 'confirming' instance of the theory or an 'anomaly' to be solved at a later date. Any ontology that was logically incoherent can be made coherent without affecting its identity.

All of this is familiar to proponents of sophisticated conventionalist and evolutionary epistemologies. I want to suggest that competing ontologies be epistemically evaluated according to the criteria of Lakatos' evolutionary epistemology; the methodology of scientific research programmes. For Lakatos [1978a: ch. 1], an explanatory theory is to be evaluated via the history of the research programme within which it is embedded. A research programme is composed of a 'negative heuristic' and a 'positive heuristic'. The 'negative heuristic' specifies the 'hard core' of the programme; its metaphysical foundations or conceptual framework. In the context of this essay, the 'hard core' consists of some version of monism of dualism. According to Lakatos, this 'hard core' is deemed irrefutable by the methodological fiat of the programme's proponents. Every worthwhile research programme develops in an ocean of anomalies. It is the function of the 'negative heuristic' of the programme to prevent such anomalies from refuting the 'hard core' by directing the scientists' attention to the revision of the 'protective belt' of auxiliary hypotheses and initial conditions. Just how the 'protective belt' is to be modified is specified by a partially articulated plan; the 'positive heuristic'.

Book cover: Metaphysics: A Very Short Introduction by Stephen Mumford

A research programme was regarded by Lakatos as 'progressive' if the successive modifications to its protective belt satisfy the following two conditions. Firstly, each successive modification must be 'theoretically progressive', or have 'excess empirical content' in the sense that the new theory, which consists of laws of nature, auxiliary hypotheses and initial conditions, must predict some hitherto unexpected, novel fact. Secondly, the modifications must be 'empirically progressive' in the sense that the predicted novel facts must be at least occasionally corroborated. Conversely, a programme that is not 'progressive' is deemed 'degenerating'. Lakatos had considered that for a research programme to be 'scientific' it must be at least theoretically progressive. And for one research programme to supersede a rival, it must be progressive while its rival is degenerating and must satisfactorily explain the previous predictive successes of the rival.[3]

Seeing ontological theories in this historical context as competing research programmes bestows significant advantages. Judging them on the basis of their evolutionary development helps clarify their present epistemic status and indicates the directions in which the competing theories should develop. In this vein, the difficulties that I will present here for the identity theory are not logically sufficient reasons for rejecting the theory, but outstanding empirical problems that stand in the way of currently accepting the theory.


  1. [1] That the reduction is one way does not logically prevent the theory from being an identity theory. For example, tables, on present physical theory, can be reduced to cohering collections of molecules structured into a particular (tablelike) shape, yet it remains true that the expressions 'table' and 'cohering collections of molecules structured into a particular (tablelike) shape' corefer to the same entity. The reduced referring expression is not eliminated.
  2. [2] For an argument against idealism, see my Allan [2016a].
  3. [3] Elsewhere, I have modified and further developed Lakatos' methodology of scientific research programmes. I have attempted to give reasons for accepting a slightly modified version of Lakatos' epistemology by characterizing the demands of a general objectivist epistemology and demonstrating how Lakatos' criteria for theory appraisal satisfy these demands. See my Allan [2016b].

Copyright © 2016

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