Are There Psychophysical Laws?
This question has been much debated since the publication of Davidson’s 1970 article “Mental Events”.Here I will give my current take on the question. First, we must distinguish strict laws from statements that are lawlike or law-ish, i.e., those that have some nomological force but don’t achieve the status of basic laws of nature. A strict law is fixed, inexorable, universal, and exceptionless; it ensures that “a particular natural or scientific phenomenon always occurs if certain conditions are present” (OED)—with special emphasis on “always”. A non-strict law (better to say “generalization”) is only probabilistic or ceteris paribus—not inexorable and universal. Our question, then, is whether there are any strict psychophysical laws, not generalizations with some degree of nomological force. What would be an example of a strict law? The question might be debated, but prime candidates would be Newton’s law of gravitation, or his three laws of motion, or the laws governing electromagnetism (Lorentz’s force law, Ampere’s law, Faraday’s law of induction, Lenz’s law: you can look them up if you don’t know them by heart). Let’s focus on the gravitational law (it is familiar and intuitive): the mass of the sun (say) acts on the earth over the intervening space and the degree of force it exerts is proportional to that mass and the distance separating the two bodies (the shorter the stronger). What is important is that the law operates continuously, reliably, and without exception—there is no deviation from it. Mass and distance are the only variables involved, and nothing can interfere with the law’s operation—things always work that way. Physical forces are like that: they never let you down, never slacken, never permit exceptions. So, the question is whether psychophysical generalizations are like that—have that kind of regularity, uniformity, simplicity. Consider first belief formation—in science, politics, ethics, aesthetics, etc. Do people believe the same things when confronted by the same evidence? Does the stimulus constituted by the evidence invariably lead to the same belief as response? It does not. No way, no how. The reason is that people form their beliefs based on a variety of factors apart from the objective evidence: their other beliefs, their desires, their personality, what they had for breakfast—and so on. Does everyone arrive at the theory of evolution by natural selection just by looking around at the animals of the world? Of course not; Darwin only arrived at it because he had vastly more evidence than other people, was more intelligent, had greater intellectual integrity, etc. We all live in basically the same world but we form very different beliefs about it. There is no strict law relating sensory input (whatever that is) and beliefs formed; it’s not like the sun’s mass and the earth’s consequent motion. The same physical stimulus does not invariably lead to the same belief formed. The process is more holistic than that, more context-dependent, messier. It is a whole lot more complex. We can’t single out a specific factor and move from that to a prediction about what will happen belief-wise. This is obvious, and it shows that there are no psychophysical laws of that type. Whoever thought there were? But what about perception—are there strict laws governing perception? Here things become less obvious but the consensus is that perception is a lot more like belief formation than has been traditionally supposed: there is no simple automatic transition from physical stimulus, distal or proximal, to eventual conscious percept. I won’t go into the details but we now know that an elaborate quasi-inferential process is at work that supplements and modifies the physical input to the senses. This process is subject to error, breakdown, individual variation, and outright illusion. There is no strict law leading from physical stimulus to veridical perceptual response; there is only the rough generalization “People generally see what’s in front of them”. Even perceptual constancies can be easily disrupted (as psychologists have shown). Not for nothing have perceptual psychologists described perception as a species of hypothesis construction analogous to scientific inference. The perceiver makes an enormous contribution to what is seen; it isn’t a kind of immediate imprinting on the senses—analogous to the influence of the sun’s mass on the earth’s motion (or the effects of magnetism). The earth doesn’t have to interpret the sun’s impact on it in order to know how to move; it’s more like the patellar or blink reflex. Perception is thus like belief formation in respect of its etiology (though not its propositional character) and unlike gravitational influence. Hence there are no strict perceptual psychophysical laws. The causal lines are far more complex and susceptible to subversion; it is not a matter of a single force uniformly doing its thing. The same is true on the output side, and more widely recognized: specific beliefs and desires don’t lead inexorably and universally to a given action; the mind gives rise to behavior in a much more holistic fashion. Again, I won’t labor the point: causal holism is generally accepted where intentional rational action is concerned. But that means that there is no strict law linking specific belief-desire pairs to action types, so there is no strict psychophysical law of this kind either (a fortiori, one might say). The causal sequence is far more circuitous and easily subverted than in cases like gravitation and electromagnetism (we have nothing like “Like poles repel and unlike poles attract”). At best we have probabilities and rules of thumb, not rigid rules and hard determinism. The interface of mind and action is as friable and elastic as the interface between stimulus and percept, more so. Let me sketch an analogy to biology: are there any biophysical laws? Specifically, are there such laws linking survival with physical conditions in the environment, as in the case of shark survival and physical conditions of the ocean? It is well known that temperature and other physical parameters affect plankton proliferation, which affects sardine populations, which affect tuna populations, which affect shark diet (they eat tuna), leading to higher or lower survival rates in sharks. So, we can say that physical conditions in the oceans cause sharks to survive in high numbers or low numbers, as the case may be. But obviously there is no strict law linking these two things, because the intervening variables are so numerous and susceptible to outside influences. We have a huge web of interconnected causal factors at work in producing shark survival or its opposite. Well, the causation of action by the mind is a bit like that: the “stimulus” afforded by a particular belief-desire pair is filtered through a vast network of other mental states that operate in concert to produce a particular behavioral “response”. We don’t have an isolated force capable of producing effects without the cooperation of other variables. In fact, this is the normal state of things: many factors combine to produce particular effects, unlike the pure cases exemplified by gravity and electromagnetism. It would be strange if psychophysical generalizations had the single-minded simplicity of basic physical forces. So, we shouldn’t be surprised if it turns out that strict psychophysical laws are not to be found in nature. Causation is seldom simple and straightforward, and it attains a high degree of complexity in the relations between mind and physical reality. Gravity and electromagnetism are special cases: they obey laws that really are strict and unbending, tunnel-visioned and obsessive-compulsive, uncooperative and go-it-alone. Strict laws have a localized causal structure, but the causal structure of psychophysical generalizations is global and diffused—multi-causal not uni-causal.
 I wrote about it in my 1978 paper “Mental States, Natural Kinds, and Psychophysical Laws”.
 Notice the law of gravitation doesn’t say that some global state of the sun is the cause of the earth’s motion; it says that mass is, and mass alone. But in the case of the mind many causal factors are at work simultaneously.
 This way of looking at things does make the denial of psychophysical laws seem unsurprising and rather banal, not bold and exciting (as it perhaps seemed to Davidson—and to my earlier self). Still, it has the advantage of being demonstrably true (Davidson always admitted that his arguments against psychophysical laws were less than conclusive). Strict covering laws are the exception rather than the rule in the universe as we have it.