Non-reductive physicalism
The earliest forms of physicalism, growing historically out of materialism, were reductionist. But after
Donald Davidson introduced the concept of
supervenience to physicalism, non-reductionist physicalism became more popular.
Non-reductive physicalism is the idea that while mental states are physical they are not reducible to physical properties. Donald Davidson proposed
anomalous monism as a non-reductive physicalism. Supervenience physicalism (also proposed by Donald Davidson) is a non-reductive physicalism, as mental events
supervene (i.e. physical properties are identical to mental properties) on physical events rather than mental events
reducing to physical events. For example if we accept supervenience physicalism, the pain someone would feel if electrocuted would supervene on the
firing of their
c-fibres. If we accept, reductive physicalism, the pain would be those c-fibres firing.
Emergentism is a theory which came to popularity in the early twentieth century. It is a form of non-reductive supervenience, but one where reality is considered to supervene in a manner more akin to layers, rather than patterns within a single layer, as per later physicalism. These layers are said to be genuinely novel from each other (i.e. the psychological vs. the physical), and is thus a type of
dualism. Physicalism is essentially
monistic.
Nonreductive physicalism has been especially popular among philosphers of biology and some biologists, who argue that all biological facts are fixed by physical facts but that biological properties and regularities supervene on so many multiple realizations of macromolecular arrangements that the biological is not reducible to the physical. Prominent exponents of this view are Philip Kitcher and
Elliot Sober.
Alexander Rosenberg introduced Davidson's notion to the debate in 1978 but thereafter argued against nonreductive physicalism in ways similar to Jaegwon Kim's (see immediately below).