I share here a few passages drawn from Roger Trigg, Beyond Matter: Why Science Needs Metaphysics (West Conshohocken, PA: Templeton Press, 2015), which I read and marked up some time back. Dr. Trigg, the founding president of the British Philosophical Association, was a student of the late A. J. Ayer, one of the principal thinkers of the school of “logical positivism.” He was, at the time the book was published, a professor emeritus of philosophy at the University of Warwick and a senior research fellow at the Ian Ramsey Centre of the University of Oxford:
It is ironic that determinist accounts that reduce everything to physical terms are confronted with the fact that physics itself seems indeterminist at the micro level. (105)
Physicalism and reductionism go together. If everything can be translated into the language of physics, concepts particular to other scientific disciplines, such as biology, will be in the last resort redundant. The unity of science becomes the triumph of physics. Reductionism is indeed an example of a scientific imperialism, by which one science arrogates the right to subsume other sciences under it. (105-106)
The kind of reality envisaged by physicalists cannot allow for the independent reality of mind, belief, or reason. All is in physical form and physically determined. Everything is seen as governed from the bottom up. Any other influences would be beyond the scope of physics and, hence, would be dismissed by a consistent physicalist whose idea of the unity of science is dictated by the need for physics to set the terms of the debate.
Beliefs in the unity of science, and in any concomitant physicalism, have themselves to be rationally grounded and not the result of blind physical processes. If metaphysical speculation, philosophical reasoning, and scientific belief are merely identical with the firing of neurons, we lose the power of rationality to discover what is true. (120)
Arguments about the dualism of mind and body must be a major issue for physical reductionism. Reductionism is itself a theory produced by human minds, and the danger is that one saws off the branch where one sits. Reduction of the mental to the physical may remove the possibility of giving rational grounds for the basic position. This itself is not a scientific matter to be solved by more experiments, but again it has to be a philosophical problem, even a metaphysical one. (122)