I argue that satisfaction and reference—and therefore, truth—are multiply realizable properties. I advocate a novel approach motivated by a commitment to the robustness and fruitfulness of truth-theoretic approaches to natural language semantics. DEFLATIONISM: Philosophers keen on deflating the metaphysical pretensions of truth theories claim that we need not appeal to a substantive truth-property. Recently, however, some philosophers have sought to combine deflationism about truth with the view that our concept of truth or the truth-predicate can play an important role in natural language semantics. TRUTH-THEORETIC SEMANTICS: The goal of a formal semantic theory of a natural language is to provide both the semantic values of that language’s lexically primitive items as well as the semantically significant modes of combining those basic elements into meaningful and more complex expressions. Most approaches have in common a commitment to finite stateability and compositionality as well as a commitment to something like Davidson’s “Convention T.” PLURALISM: Pluralists about truth argue that different areas of discourse have different truth-properties. Can pluralism successfully be combined with a commitment to truth-theoretic semantics? OPEN SEMANTIC FUNCTIONALISM: The pluralist approaches to truth are unsatisfactory for a variety of reasons. The only option, I argue, is to regard truth as multiply-realizable. Specifically, we should view the set of truth’s realizers as possessing non-actual members—as being “open.” Truth is defined in the usual way in terms of reference and satisfaction, but these latter two relations are to be understood as multiply realizable but open. The property of truth can be specified using the Ramsey/Lewis method. My final view—Open Semantic Functionalism—respects compositionality and finite stateability, avoids triviality, handles plurality, and fits with robust, explanatorily significant natural language semantic theories.

Type identity theory is a theory within physicalism that maintains that types of mental states or processes are identical to types of states or processes in the brain. This would imply mental types can’t be encountered in physical structures other than the brain. But it seems to be the case that other biological creatures with different physical processes going on in their brain are able to experience a mental state such as pain. Even our brain, with its ability to undergo modification, functionally and structurally, in response to experience or injury, an ability called neuroplasticity, seems to have the ability to instantiate or realize the same mental state by two different neural events. This is the multiple realizability nature of the mental, that challenge type identity theory, but that functionalism seems to account for. Multiple realizability seems to provide a plausible basis for the irreducibility of mind to brain, and thus a good argument for why they should not be considered identical. I argue that when type identity theory claims that types of mental states or processes are identical to types of states or processes in the brain, this should be understood as claiming that types of mental states are identical to types of mechanisms in the brain, by which I mean types of interaction between fundamental particles that comprises the brain. I argue that two physical structures realizing the same mental state must be relevantly similar in the way their respective fundamental particles are interacting with one another. In other words, that similarity in function implies similarity in the way the physical realizers are organized. Therefore, the multiple realizability nature of mental states shouldn’t render them irreducible or non-identical to specific types of interactions fundamental particles.

It has been argued that there are physically different ways of instantiating mental properties, the nature of which is the subject matter of cognitive science. This claim has been known as the Multiple Realizability Thesis (MRT). It has been suggested that the MRT shows that a reduction of mental properties to physical properties is impossible, as there cannot be one-to-one correspondences of mental properties to the properties of the brain. Moreover, it has been argued that the latter point shows that physical explanations are not relevant to the explanations of cognitive science, as they would lack the generality of psychological explanations. This thesis will try to explain from which assumptions of a traditional cognitive science perspective the MRT follows. It will also discuss several responses that have been introduced against both the MRT and the anti-reductionist conclusions that are assumed to follow from it. The responses include a challenge to the scientific status of cognitive science. According to this challenge, the MRT entails that the subject matter of cognitive science, namely mental properties, lack a similarity in the physical level, hence an instance of a mental property is not informative about another instance. While discussing these theories, a revision of the MRT will be proposed. According to this revision, the MRT is compatible with the assumption that there could be an underlying similarity between different physical realizers of a given mental property. It will be argued that by means of this revision, both the challenge to the scientific status of cognitive science, and the argument for the irrelevance of physical explanations will fail.