According to Searle, the two theses of BN work within a micro-macro schema that is compatible with the current scientific world-view. Using the micro-macro model, we can formulate the theses (combined) in another way by saying that minds and mental states are the higher-level phenomenon of the lower-level phenomenon that is physical brain processes. For example, heat is the higher-level phenomenon of the lower-level phenomenon that is particles/atoms moving at a very fast rate. Similarly, pains are the higher-level phenomenon of the lower-level phenomenon that is a certain pattern of neuron firings. So, the lower-level phenomenon causes the higher-level phenomena in that the later is realized in and consists in the former. Thus, we have the first thesis: brains (physical processes inside the brain) cause minds (mental states, consciousness, etc…).
As for the second thesis, let us refer to the heat example above. Suppose we have a pile of H2O molecules. If the molecules act a certain way (i.e., slow down), the pile as a whole becomes “cold.” If, on the other hand, they act in another certain way (i.e., speed up), then the pile as a whole becomes “hot.” Thus, depending on the physical processes occurring within the pile of H2O molecules, the pile can acquire the feature of either “heat” or “cold.” Similarly, mental states are features of brain processes. If my neurons act in a certain way (i.e., get shot to a certain place), then my person (mind) as a whole experiences a certain mental state. This mental state is thus a feature of the brain, just as heat is a feature of the sped-up pile of particles.
Searle’s solution faces numerous challenges, of which a fundamental one is articulated by Nagel. For Nagel, it is not possible to have an account of the mind-body relationship that is also compatible with the current scientific world-view. In science, as Nagel points out, causal relationships entail a necessity in the relationship: given A, it is necessary that B (the lack of B is inconceivable). For example, given the information that H2O molecules in the pile are speeding up, one necessarily concludes that the molecules are generating heat. But according to Nagel, no account of mind-body relationship can achieve this necessity in the relationship. This is so because given a certain set of brain processes, it is not necessary that a certain mental state accompany this process. While it is inconceivable to have sped-up particles without heat, it is conceivable to have certain brain processes without certain mental states. So, interaction between mind and matter is arbitrary. Due to this notion of necessity, Nagel believes that there can be no account of the mind-matter relationship.
In response to such an objection, Searle has several points to make. First, Searle points out that not all science involves a necessity clause that explains why something in the physical world necessarily happens. For example, quantum mechanics is a theory of the physical world that works randomly rather than necessarily, and hence is not dictated by the necessity clause. Since it attacks one of Nagel’s assumptions, that science relationships entail necessity, this seems to be a good response. On the other hand, it does not adequately address the concern of why mental states are necessarily a feature of brain processes. At this point, Searle makes a second argument: that necessity is a social construct. We today may think it inconceivable to have H2O molecules moving very quickly but without generating heat. However, a person from another time, say 3000B.C, may not agree with us on the necessity within such an explanation of H2O molecules and heat. Thus, the notion of necessity itself is not necessary.
While Searle’s response brings up some valid points about the idea of necessity, I do not believe that it is a successful reply to Nagel’s objection. Ultimately, Nagel’s objection comes from a Cartesian, dualistic tradition of mind-body separation. It is based upon this understanding of a gap between mind and matter that Nagel is debunking Searle’s theory which does not seek to bridge any gap, much less concern itself with necessity. But Searle’s theory begins by rejecting the Cartesian vocabulary, and ultimately, the Cartesian world-view itself. In at least this aspect, Searle and Nagel do not start with the same premises, and this nullifies any chance for a substantial argument. Thus, albeit trivially, Searle’s reply does not successfully answer Nagel’s objection.
