Jaegwon Kim claims that Biological Naturalism is a causally overdetermined description of the mind. Basically, he notes that BN holds that all mental states are caused by neuronal processes while some of them are also caused by previous mental states. A mental state M1 a pain, for example, causes M2 a desire to take asprin. According to BN, both of these mental states must be caused by brain states, B1 and B2. Therefore, Kim says, M2, being caused by M1 and B2, is then causally overdetermined. Kim says that BN can escape causal overdetermination only by accepting identity statements such that M1 = B1 and M2 = B2. He sees that to make the BN system work, there must be only one instance of causation. His reasoning is that all instances of causation happen over time between two discreet events such that the prior event causes the later event. If M1 is not equal to B1, then the system will be faced with causal overdetermination or it will describe two parallel causes that have nothing to do with each other. In other words, BN needs to say that 1)“M1 causes M2” = “B1 causes B2”. Kim thinks the only way to do this is to say that 2)M1 = B1 and M2 = B2. Searle, in defending BN, will agree with 1, but not with 2.
John Searle defends BN in an unpublished letter to Kim. He writes that Kim is working with obsolete assumptions concerning causation. Kim has inherited from Hume the belief that all causation occurs over time between two discreet events, but this is not true. For example, the molecules of a hammer cause it to be solid, both of which happen simultaneously. When B1 causes M1, the causation is of this latter form. Searle calls this a bottom up cause. Causes of this form are usually said to be explanations rather than causes because the events involved are identical. In this way, Searle maintains that M2 does not really have two causes. One of the reported causes is actually an explanation.
Searle does not say that Humean causation never occurs. He only denies that it explains every instance of causation. According to BN, M1 causes M2 is an instance of Humean causation, but that that instance of causation can be described from a different level, namely B1 causes B2. So, Searle agrees with sentence 1 above. However, he rejects sentence 2. Since M1 is causally reducible it B1, the two act causally as one event, but this does not entail that they themselves be equal. Searle describes an example to explain this. Searle often walks around the classroom while he lectures. The event in which he walks is identical to the event in which he lectures. However, lecturing does not equal walking. This is the meaning of the phrase “causally reducible yet not ontologically reducible.”
Kim was right in his assessment of BN when he said that mental causation must be reducible to a single instance of Humean causation. However he was wrong to say that the only way to achieve this is to accept ontological reductions like those seen in token-identity. Searle was misleading in his original description of BN when he said that brain states cause mental states. At the least, this causation is of a different form from usual Humean causation.
Searle saves BN from causal overdetermination, but in doing so he opens new problems. To say that mental states are causally reducible to brain states is to say that anything that causes M1 for instance, also causes B1 and anything caused by M1 is also caused by B1. But now, what about the causal claim that B1 causes M1? Searle claims that the causal story of “B1 causes M1” is the same as “a certain molecular structure causes solidity,” but this seems odd since molecular movement is identical to solidity and brain states are not identical to mental states. In Searle’s letter to Kim, he adamantly resists that M1 = B1 insofar as that claim is taken to be an ontological reduction. That M1 is causally reducible to B1 helps us understand how the two function as a unit in causation that occurs over time (left to right causation), but this tells us nothing about how B1 causes M1 (bottom up causation). This is not necessarily an objection to BN, but a call for further explanation.
The biggest problem for BN as elaborated in Searle’s letter to Kim is that it leaves absolutely no room for freewill. The mental and physical both act as one event according to causal laws. If the physical world is causally closed, where is there room for the indeterminacy in which freewill can operate? Searle describes freewill as a gap in the causal structure of mental states. One could have all the same beliefs and desires and yet choose to act differently. One mental state does not necessitate the cause of another mental state. But, according to BN, the causal powers of a mental state are the same as the causal powers of the brain state. The causal powers of brain states are readily understood by physics and neurology. There is no gap in the causal structure of brain states. Searle himself maintains that the physical world is causally closed. That BN leaves no room for freewill is not a form of epiphenomenalism. Mental states are able to cause other mental states just fine, but this description leaves no possibility that the mental states be a part of any non-necessary cause. That the causal powers of a mental state are identical to the causal powers of the accompanying brain state saves BN from causal overdetermination, but this very fact makes it incompatible with a theory of free will.