The program of reductive physicalism has as its aim the reduction of the theory of the mind to a physical theory. This means that they intend to explain all aspects of the mental in terms of physical phenomena, and use the physical phenomena to 'define' the corresponding mental events.
Given that mental and physical theory are conceptually separate and have very different basic precepts, this reduction requires the use of 'bridge principles' that map concepts from one theory to the other. These bridge principles can either be in the form of definitional relationships or empirical facts. For bridging the mental and physical, definitional relationships are essentially useless. The behaviourists found it impossible to create such relationships based on observable aspects of behaviour. Thus, we must search for empirical bridge principles.
Modern cognitive neuroscience concerns itself with finding these bridge principles by studying the action of the brain under controlled circumstances. These experiments have found a number of possible bridge principles, which identify simple mental events with activation particular features in the brain. From a reductionist perspective we can say that these activations 'explain' the mental events.
A major issue with reductive physicalism is multiple realisability. It is clear that both humans and octopuses experience pain, and equally clear that the brains of humans and octopuses are very different. Thus if we identify pain with a particular feature of the human brain, we must conclude that the octopus isn't actually experiencing pain. The reductive physicalist would respond to this by claiming that these empirical bridge principles are contingent on the species of the subject. Thus it is reasonable to expect that there would be a different mapping for an octopus, than for a person.
It remains possible that these empirical mappings of mental states to physical events are different, not only for different species, but for different individuals within a species. This would preclude the existence of a reductive theory that is both perfect and general. However, this does not prevent the creation of a theory that is general and useful. Although the physical workings of each human are different, medical science produces many useful results with an averaged perspective. The efficacy of this method does, however, depend on the range of these variations.
Node your homework
This writeup is copyright 2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .