Emergentism. A property of a system is said to be emergent if it is more than the sum of the properties of the system’s parts.
Physicalism may be defined as the theory that the universe is comprised exclusively of physical entities. However, biological systems and consciousness, for example, appear to be problematical for this thesis, as they exhibit properties not ordinarily associated with most other physical entities. In response to this situation, two variants of physicalism have been advanced: reductionism and emergentism.
Reductionists generally see the task of accounting for the possibly atypical properties of mind and of living things as a matter of showing that, contrary to appearances, such properties are indeed fully accountable in terms of the properties of the basic particles of nature (or in other, like terms), and therefore in no way genuinely atypical. By contrast, emergentists have argued that what is meant by the physical is more complex than this picture suggests, and that novel properties can arise above the level of fundamental particles. Thus, emergentism suggests a layered view of nature, with the layers arranged in terms of increasing complexity with each requiring its own special science.
Some philosophers hold that emergent properties causally interact with more fundamental levels, while others maintain that higher-order properties simply supervene over lower levels without direct causal interaction. The latter group therefore holds a definition of emergentism which can be stated as follows:
a property P of composite object O is emergent if it is metaphysically possible for another object to lack property P even if that object is composed of parts with intrinsic properties identical to those in O and has those parts in an identical configuration.
This purely metaphysical account, is by no means the only one possible, and certainly not the most plausible, would seem to lack physical examples.
C. D. Broad provided an entirely different definition:
Put in abstract terms the emergent theory asserts that there are certain wholes, composed (say) of constituents A, B, and C in a relation R to each other; that all wholes composed of constituents of the same kind as A, B, and C in relations of the same kind as R have certain characteristic properties; that A, B, and C are capable of occurring in other kinds of complex where the relation is not of the same kind as R; and that the characteristic properties of the whole R(A, B, C) cannot, even in theory, be deduced from the most complete knowledge of the properties of A, B, and C in isolation or in other wholes which are not of the form R(A, B, C).
The first emergentist theorists used the example of water having a new property when hydrogen, H, and oxygen, O, combine to form H2O (water). In this example there emerge such new properties as liquidity under standard conditions. (Analogous hydrides of the oxygen family, such as hydrogen sulfide, are gases). However, a better and more recent example of an emergent phenomenon, one provided by physicist Erwin Schr dinger, is found in the case of families of molecules known as isomers, which are made up of precisely the same atoms, differently arranged, which nevertheless have different physical properties. Similarly, enantiomers are molecules made up of precisely the same atoms, in precisely the same arrangement, but which exist in “right-handed” and “left-handed” forms, and also have different properties when interacting with other molecules.
Biologists Ursula Goodenough and Terrence Deacon in their 2006 essay on emergencehave assembled a range of examples of physical and biological emergent properties that provide the evidential basis for emergentism as a philosophy that comports with a modern scientific understanding of how complexity arises in the natural world, and as a philosophy that supports religious naturalism. A longer compilation of emergent forms in nature is the 2004 book by biologist Harold Morowitz: The Emergence of Everything.
Emergentists have suggested that the mind-body problem is better accounted for in emergentist terms. Jaegwon Kim, by contrast, has said that a problem for emergentism is “causal closure” in a universe that does not allow for a mind-to-body causation. Emergentists, however, do not see the mind as something different from “the body” - but rather as something with properties uniquely its own. If these properties can exercise downward causation, it may be that there is no problem here. (This issue remains especially controversial.) However, the problem of mind-body interaction is at least as serious a problem for reductive physicalism, leading many reductive physicalists to deny the very existence of mind through a lack of alternatives.