The Mind-Brain Type Identity Theory and Multiple Realizability

2024-1 Seminar Metaphysics

 

1. Introduction

In his pioneering paper, “Sensations and Brain Processes,” J. J. C. Smart developed and relied on the so-called Mind-Brain Type Identity Theory, which was once considered the most dominant and promising candidate for shedding light on the nature of our minds. In the present paper, I aim to figure out the reasons why Smart’s type identity theory appealed to so many philosophers and subsequently lost its explanatory power. As is widely known, the most serious objection to his theory is called Multiple Realizability Arguments, originally posed by H. Putnam. Accordingly, I will address questions such as how Multiple Realizability undermines Smart’s thesis. Finally, I will examine an attempt to reinterpret the problem of Multiple Realizability, challenging the Individuation of Mental Kinds (Bickle 2022).

2. Smart’s Mind-Brain Identity Theory

Starting his discussion, Smart straightforwardly clarifies the motivation for constructing his theory, which is widely accepted even nowadays, as follows:

And similarly the suggestion I wish to resist is also that to say “I have a yellowish orange after-image” is to report something irreducibly psychical. Why do I wish to resist this suggestion? Mainly because of Occam’s razor. [...] That everything should be explicable in terms of physics (together of course with descriptions of the ways in which the parts are put together-roughly, biology is to physics as radio-engineering is to electromagnetism) except the occurrence of sensations seems to me to be frankly unbelievable (Smart 1959: 142).

Smart’s concern about mental images, often characterized as a tension between manifest and scientific images (as described by Sellas [1991]), is twofold. First, he maintains that any attempt to explain the nature of our mental images should be compatible with modern physics, such as Newtonian mechanics. This belief is grounded in the underlying assumption that any object dealt with in any discipline should be describable at the most fundamental level, from the standpoint of physics. On the contrary, positing something irreducibly psychical renders mental phenomena inscrutable and mysterious. It seems far-fetched to suggest that the laws of physical reduction apply universally except when it comes to mental entities.

Moreover, this reductionist view is bolstered by the application of Occam’s razor. As explained by Baker (2022), this principle asserts that the fundamental laws underpinning a theory should not be multiplied unnecessarily. In other words, a compelling theory ought to comprise fewer, yet essential laws. In this vein, it becomes simpler to account for all phenomena under a unified framework of physical laws and properties, as opposed to postulating unique laws for psychological phenomena that do not belong to the laws of physics.

Now, let me encapsulate Smart’s motivation for constructing his identity theory as follows: it is an attempt to reconcile a mental image with a physical worldview by naturalizing the former and maintaining the simplicity of the latter. In this regard, Smart argues that sensations are identical to brain process in the sense that every sensation statement is a report of a brain process (Smart 1959: 145). It is worth noting that such a relation is a metaphysical identity, which entails that “Sensations are nothing over and above brain processes” (Smart 1959: 145; Polger 2011: 2-3). If we acknowledge such an identity, it follows that something like the Cartesian ego cannot possibly be established at all. Polger regards this claim as the most valuable legacy of Smart’s work, stating: “the insistence that sensations are (in some sense) nothing over and above brain processes is now commonplace, as a basic component of any generic physicalism about the mind” (Polger 2011: 2-3).

3. Multiple Realizability

If Smart’s theory is logically consistent and aligns with all the evidence we have, it would probably be the simplest theory of the mind. Unfortunately, Putnam has demonstrated that our empirical evidence seriously conflicts with the identity theory. According to Type Identity Theory, one must conclude that beings with a physical system different from humans, such as those without C-fibers, cannot experience pain at all; for being in pain is exactly the same as one of our physical processes. However, it seems undeniable that creatures like octopuses can also feel pain, despite lacking C-fibers. This suggests that our mental states or processes are multiply realized or realizable (Polger 2011: 8). Therefore, Type Identity Theory and multiple realizability are incompatible, and it seems quite natural to intuitively reject the former.

Historically, this argument has provided a decisive reason for many philosophers to abandon the identity theory and advocate for functionalism. In a nutshell, why do we call a specific psychological state in both an octopus and myself ‘pain’ if they are realized by different physical states? The classic answer to this question is that these instances of pain serve the same function.

However, it is still questionable whether a single given kind is truly multiply realized (Bickle 2022). For instance, Lawrence Shapiro points out that the concept of realization itself contains ambiguity. According to this view, we appeal to the sameness of function when (intuitively) justifying Multiple Realizability Thesis, while this appeal seems to undermine the claim that such realizers are different realization of a particular kind of thing. Take, for example, two corkscrew made of steel and aluminum, respectively. Shapiro argues that two corkscrews, alike in constitution and mechanism but distinct in color, cannot count as alternative realizations of the kind corkscrew, for

the only property by which they differ-color-is not a property that contributes to their capacity to remove corks. To say that a kind is multiply realizable is to say that there are different ways to bring about the function that defines the kind. But, if two particulars differ only in properties that do not in any way affect the achievement of the defining capacity of a kind, then there is no reason to say that they are tokens of different realizations of the kind. (Shapiro 2000: 643-644)

If A and B are indeed different multiple realizations of a particular kind of thing, it is necessary that the causally relevant properties of A and B differ (Shapiro 2000: 644). However, “if they do so differ, then they genuinely are different kinds. […] and again we don’t have an instance of multiple realizability” (Bickle 2022).

References

Baker, A. (2022), “Simplicity,” The Stanford Encyclopedia of Philosophy (Summer 2022 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/sum2022/entries/simplicity/>.

Bickle, J. (2022), “Multiple Realizability,” The Stanford Encyclopedia of Philosophy (Summer 2020 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/sum2020/entries/multiple-realizability/>.

Polger, T. W. (2011), “Are Sensations Still Brain Processes?” Philosophical Psychology, 24(1) 1-21.

Shapiro, L. A. (2000), “Multiple Realizations,” The Journal of Philosophy, 97(12): 635–654.

Sellars, W. (1991), “Philosophy and the Scientific Image of Man,” in: Science, Perception, and Reality, Atascadero, California: Ridgeview Publishing Company. 

Smart, J. J. C. (1959), “Sensations and Brain Processes,” The Philosophical Review, 68(2), 141-156.