George Henry Lewes coined the term emergence (Lewes 1875). He drew a distinction between emergents and resultants, a distinction he learned from John Stuart Mill. In his System of Logic (1843), Mill drew a distinction between "two modes of the conjoint action of causes, the mechanical and the chemical" (p. xviii). According to Mill, when two or more causes combine in the mechanical mode to produce a certain effect, the effect is the sum of what would have been the effects of each of the causes had it acted alone. Mill's principal example of this is the effect of two or more forces acting jointly to produce a certain movement: the movement is the vector sum of what would have been the effect of each force had it acted alone. According to Mill, two or more causes combine in the chemical mode to produce a certain effect if and only if they produce the effect, but not in the mechanical mode. Mill used the term chemical mode because chemical agents produce effects in a nonmechanical way. Consider a chemical process such as CH4 + 2O2 → CO2 + 2H2O (methane + oxygen produces carbon dioxide + water). The product of these reactants acting jointly is not in any sense the sum of what would have been the effects of each acting alone. Mill labeled the effects of two or more causes acting in the mechanical mode "homopathic effects," and effects of two or more causes acting in the chemical mode "heteropathic effects." Lewes called heteropathic effects emergents and homopathic ones resultants (McLaughlin 1992).
Mill's work launched a tradition, British Emergentism, that flourished through the first third of the twentieth century (McLaughlin 1992). The main works in this tradition are Alexander Bain's Logic (1843), George Henry Lewes's Problems of Life and Mind (1875), Samuel Alexander's Space, Time, and Deity (1920), Lloyd Morgan's Emergent Evolution (1923), and C. D. Broad's The Mind and Its Place in Nature (1925). There were also prominent American emergentists: William JAMES, Arthur Lovejoy, and Roy Wood Sellars; and in France, Henri Bergson developed a brand of emergent evolution (Blitz 1992; Stephan 1992). In the 1920s in the former Soviet Union, the members of the Debron School, headed by A. M. Debron, spoke of the emergence of new forms in nature and maintained that the mechanists "neglected the specific character of the definite levels or stages of the development of matter" (Kamenka 1972: 164).
Alexander (1920) spoke of levels of qualities or properties, maintaining that "the higher-level quality emerges from the lower level of existence and has its roots therein, but it emerges therefrom, and it does not belong to that lower level, but constitutes its possessor a new order of existent with its special laws of behavior. The existence of emergent qualities thus described is something to be noted, as some would say, under the compulsion of brute empirical fact, or, as I should prefer to say in less harsh terms, to be accepted with the 'natural piety' of the investigator. It admits no explanation" (1920: 46). Morgan (1923) connected the notions of emergence and evolution and argued for an evolutionary cosmology. Morgan maintained that through a process of evolution genuinely new qualities emerge that generate new fundamental forces that effect "the go" of events in ways unanticipated by force-laws governing matter at lower levels of complexity.
Broad (1925) contrasted "the ideal of Pure Mechanism" with emergentism. Of the ideal of pure mechanism, he said: "On a purely mechanical theory all the apparently different kinds of matter would be made of the same stuff. They would differ only in the number arrangement and movements of their constituent particles. And their apparently different kinds of behaviour would not be ultimately different. For they would all be deducible from a single simple principle of composition from the mutual influences of the particles taken by pairs [he cites the Parallelogram Law]; and these mutual influences would all obey a single law which is quite independent of the configuration and surroundings in which the particles happen to find themselves" (1925: 45-46). He noted that "a set of gravitating particles, on the classical theory of gravitation, is an almost perfect example of the ideal of Pure Mechanism" (1925: 45). He pointed out that according to pure mechanism, "the external world has the greatest amount of unity which is conceivable. There is really only one science and the various 'special sciences' are just particular cases of it" (1925: 76). In contrast, on the emergentist view "we have to reconcile ourselves to much less unity in the external world and a much less intimate connexion between the various sciences. At best the external world and the various sciences that deal with it will form a hierarchy" (1925: 77). He noted that emergentism can "keep the view that there is only one fundamental kind of stuff" (1925: 77). However, if emergentism is true, then "we should have to recognize aggregates of various orders. And there would be two fundamentally different types of laws, which might be called 'intra-ordinal' and 'trans-ordinal' respectively. A trans-ordinal law would be one which connects the properties of adjacent orders. . . . An intra-ordinal law would be one which connects the properties of aggregates of the same order. A trans-ordinal law would be a statement of the irreducible fact that an aggregate composed of aggregates of the next lower order in such and such proportions and arrangements has such and such characteristic and non-deducible properties" (1925: 77 - 78). Broad maintained that transordinal laws are irreducible, emergent laws because they cannot be deduced from laws governing aggregates at lower levels and any compositional principle governing lower levels.
The British emergentists intended their notion of emergence to imply irreducibility. However, they presupposed a Newtonian conception of mechanistic reduction. Quantum mechanics broadened our conception of mechanistic reduction by providing holistic reductive explanations of chemical bonding that make no appeal to additive or even linear compositional principles. The quantum mechanical explanation of chemical bonding is a paradigm of a reductive explanation. Chemical phenomena are indeed emergent in the sense that the product of chemical reactants is a heteropathic effect of chemical agents; moreover, the chemical properties of atoms are not additive resultants of properties of electrons, and so chemical properties of atoms are emergent. However, the quantum mechanical explanation of chemical bonding teaches us that reductive explanations need not render the reduced property of a whole as an additive resultant of properties of its parts. Reductions need not invoke additive or even linear compositional principles. The quantum mechanical explanation of chemical bonding, and the ensuing successes of molecular biology (such as the discovery of the structure of DNA) led to the almost complete demise of the antireductionist, emergentist view of chemistry and biology (McLaughlin 1992).
Nonetheless, the British emergentists' notion of an emergent property as a property of a whole that is not an additive resultant of, or even linear function of, properties of the parts of the whole continues to be fairly widely used (Kauffman 1993a, 1993b). The term emergent computation is used to refer to the computation of nonlinear functions (see the essays in Forrest 1991).
In philosophical circles, there have been some attempts to develop a notion of emergence, loosely based on the British emergentist notion, but that actually implies ontological irreducibility (Klee 1984; Van Cleve 1990; Beckermann, Flohr, and Kim 1992; Kim 1992; McLaughlin 1992, 1997). These attempts invoke the notion of SUPERVENIENCE and make no appeal to nonadditivity or nonlinearity. On one view, bridge laws linking micro and macro properties are emergent laws if they are not semantically implied by initial microconditions and microlaws (McLaughlin 1992). One issue that remains a topic of intense debate is whether, in something like this sense of emergence, bridge laws linking conscious properties with physical properties are irreducible, emergent psychophysical laws, and conscious properties thereby irreducible, emergent properties (Popper and Eccles 1977; Sperry 1980; Van Cleve 1990; Chalmers 1996).
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