Broad The Traditional Problem of Body and Mind
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ShaughanLavine - 09 Nov 2005
Broad's argument from the conservation of energy is a bad argument, and bad at least in part for his reasons, but there is much better argument from physics of which, so far as I can tell, the conservation of energy argument is just a weak version.
The argument is the argument from the causal closure of the physical.
What is the causal closure of the physical? Everyone starts with Laplace's demon. According to Newtonian physics, if you know the positions and velocities of every particle in the universe at one instant and the force laws, you would be able to predict the complete physical configuration of the universe at every instant. The point is usually associated with determinism, but it isn't quite the same. To the extent that a system is isolated, the same principle applies to that system.
That means, for example, that the motion of my arm is fully accounted for by the physical configuration of the universe, and, over short time periods, fully accounted for by the physical configuration of my body (plus a piece of floor, and gravity). Thus, there is no room for the mental to influenced the physical.
This argument may have fallen out of favor because Newtonian mechanics has. That isn't crazy for at least two reasons: the physical structure of the brain relies on biochemical reactions, and our chemical explanations of the forces involved reduce to quantum mechanics, but not to Newtonian mechanics, and the second (flaky) reason is that there is a bunch that claims that the way the brain functions involves macroscopic quantum correlations.
There are two ways that I know of to extend the argument to quantum mechanics. First, on standard interpretations, quantum mechanics is causally complete (though not deterministic). When quantum mechanics only predicts that something will happen with probability 50%, that is not an epistemic (ignorance) probability (which would leave room for other causal factors), but a genuine physical probability which characterizes the situation as being one in which the objective facts of the matter fully determine that the probability is irreducibly 50%, and so there is no room for anything (mind) to influence this situation any more than the Newtonian one.
There are interpretations (so-called hidden variable interpretations) of quantum mechanics such that the probabilities are merely ignorance probabilities. Unfortunately, the "variables" that determine the outcomes, in most such theories, are Laplacian, so that's no help. There is a theory in which particles just jump at random once in a while, but it really has to be, overall, at random to get the statistics right, and so letting the mind tip the particles would require an odd sort of special pleading.
There is also a so-called "many minds" interpretation of quantum mechanics, in which every possibility occurs in some parallel universe, and minds can influence (or perhaps just move in parallel with) which parallel universe they wind up in.
Here is what I take to be the best version of the causal closure argument for quantum mechanics: Newtonian mechanics permits us to predict everything with certainty given total knowledge at a moment. Quantum mechanics permits us to predict, with certainty, the outcome of any single future measurement by making a single suitable present measurement. That means that you can predict what any person will do on purely physical grounds before they make any relevant decisions. Once again, there is no room for mind, and note that this version of the argument is not sensitive to interpretation except insofar as it assumes that measurements have outcomes.
The argument from causal closure is better than any of the arguments Broad considers, and it is worth discussing what kind of constraints it imposes.
His problem is Interactionism: does the mind cause the body to react and does the body cause the mind to react?
The first important result of looking at that problem is that it is trivial if cause is just constant association. There is no issue unless one has some non-Humean notion of causation.
That is intimately connected with Broad's argument that Hume's argument against causation fails in the case of willed actions. Broad agrees that Hume is right that willing is not a sufficient cause of action, but Hume argues that a cause can never be known to be a cause of its effect, merely associated because there is no additional property for the association to have to mark causation. Broad says that that is false in the particular case of mental causation, because the intentional provides the missing link. He uses the example of moving your arm, but Annie pointed out that when your arm is asleep and you move it with your other arm, Broad's argument doesn't seem to work. (But who cares?) Hume argues that there is nothing available to serve as the missing link, and Broad correctly points out that the intentional is available to do so (at least I'm willing to grant that), but he does not argue that it actually does so.
He also argues that supposing that there is no body-to-mind causation commits us to wildly implausible hypotheses, but that supposing that there is no mind-to-body causation does not.
All of this presupposes, besides causation, that there are minds and bodies. Given that assumption we can eliminate mind-to-body causation in at least two ways. One is zombies (and the possibility that our bodies are zombies even though we haven't noticed), and the other is epiphenomenalism.