Spinoza wrote during the transitional 17th century, when the medieval use of the term “God” as a term of art in philosophy overlapped with the early modern interest in the new science, particularly the mechanistic physics that grew out of astronomy and achieved its greatest expression in the work of Isaac Newton at the end of the century. Spinoza took a particular interest in the mind-body problem within the century’s larger preoccupation with reconciling the well-ordered, necessary world revealed by logic and mathematics with the seemingly chaotic, contingent world observed by empirical science. Philosophers of this period are often accused of hiding a thoroughly modern secularism behind religious language so as to avoid trouble with the authorities and to make their modern doctrines go down more easily, but this is a specious interpretation: these thinkers were developing modern ideas out of the old and it was a confusing and difficult process. One has to learn their philosophical language and try to understand their use of the word “God” as a technical term.
Spinoza’s core metaphysical argument is that God can have no limitations: to say that there was anywhere a boundary such that God was on one side and not on the other violated God’s property of universality (notice that one can substitute “mathematics” for “God” here with no loss of sense). Spinoza concludes that the universe (he often uses the term “nature”) is identical to God. It follows that the universe is necessary and perfect. The idea is that the causal processes of nature, seemingly full of contingency and randomness, actually unfold following mathematical necessity (in this regard Spinoza’s views are very close to Newton’s). Humans cannot see this directly due to our own limitations, but we can cultivate an attitude appropriate to the insight.
From this central doctrine Spinoza developed what is commonly called a “double aspect” theory of the relationship between the mind and the body. Everything (the universe) is both the mind and the body of God. Thus everything comes under both a mental and a physical description, which are two ways of looking at the same thing. A benefit of this view is that there is no question about either mental-to-physical or physical-to-mental causation; nor is there any question of choosing between dualism, idealism or materialism: Spinoza presents a monism where the only item of ontology is “God.” Both our mental nature and our physical nature are in fact aspects of our “Godly” (that is, rational) nature. This helps to make sense of the seemingly bizarre doctrine of “synchronicity” developed by Spinoza’s successors Leibniz and Malebranche, which expresses essentially the same idea: the mental and the physical are both explained by a common, antecedent source, which also explains how they are linked (for Spinoza they are one).
Spinoza’s metaphysics leads him to the strikingly modern position of rejecting dualist language about, for example, the mind being the controlling cause of the body’s movements (the ghost in the machine). But also Spinoza rejects the idea that the physical world (the “mode of extension”) is anything random or otherwise contingent. The physical/extended world has structure that corresponds to the mental world. His insight that fine-grained physical processes in the body instantiate the fine-grained processes of the mind (the body is “the object of the mind”) is achieved not by eliminating rationality (as Hume attempts to do) but by merging the rational and the physical.
The contemporary philosopher Jerry Fodor develops a similar line. Fodor understands that physicalism requires that the non-physical property of intentionality be washed out of the ultimate account of things, but he is convinced that mental content is ineliminable: two positions that would appear to be mutually exclusive. What account of mental representation can be given that does not involve us in reference to the semantic property? Fodor proposes to translate semantic properties into syntactic properties. The syntactic structure of the proposition (that is, of the mental representation that is implicit in the intentional attribution) maps on to the computational structure of cognition, which can be cashed out at the machine-language level. A “machine-language” isn’t really a language at all, in the sense of “language” as a symbol-system with semantic content. In the case of computers, binary code (1s and 0s) represents the physical state of the electronic gates in the microchips (open or closed). As the creators of computers we can explain the words and images on the computer screen in terms of the underlying physical process.
Uncovering the machine-language of the nervous system looks like a holy grail for cognitive science. But computers are artifacts that, ultimately, move symbols around for human beings to interpret, so the computer analogy doesn’t go through: actual mental content of the sort that (as Searle demonstrates, convincingly to my mind, with the Chinese Room) computers utterly lack has to be explained without appeal to an interpreter. The mental must be explained wholly in non-mental terms. Computers have the mental already built in: their human users. Fodor’s most expensive proposal is his idea that the causal role of mental content can be explained wholly in terms of the syntactical properties of the representation: that syntax alone can perform the function of sustaining and respecting the logical entailments between the propositions. This requires that he rejects meaning holism in favor of meaning atomism: like immune system antibodies, each mental concept must be latent and autonomous. This is also necessary if we are to keep cognitive psychology (as Fodor believes we must) “in the head.”
I have already expressed my sympathies for externalism and my eliminativism regarding mental representation. However I come not to bury Fodor but to praise him. Fodor has the same insight as Spinoza: to understand the identity of the mental and the physical requires that we understand not only the mental as, in some sense, physical but also the physical as, in some sense, mental. The problem of rationality, unlike the problem of representation, will require something like the double aspect approach. Spinoza proposes the aspects of mind and body, Fodor proposes the aspects of semantics and syntax. That Fodor is a realist about representations and I am an eliminativist does not turn out to mean that we fundamentally disagree. Specifically I don’t think that syntax entails symbol entails semantics, a line of criticism that many materialists would take against Fodor. However, while Spinoza and Fodor both point the way to the resolution of the problem of rationality, to actually get there we must now consider the metaphysics of one of the greatest of all philosophers.