What is reductionism?

In order to understand the role and implications of emergence it is helpful to define different forms of reductionism in science. It is also important to make a distinction between reductionism as a practice in science and reductionism as a philosophical outlook. As a method, reductionism has been extremely powerful. Examples of successes include the understanding obtained by reducing genetics to molecular biology, atomic spectra to quantum mechanics, and planetary motion to Newtonian mechanics. A reductionist approach gave a unifying description of a diverse range of phenomena, and elucidated "cause and effect", i.e., if one component or variable of the system is changed what is the resulting change in other components or properties. In terms of popular books, advocates of the primacy of reductionism include Steven Weinberg, Stephen Hawking, and Richard Dawkins. They also appear to presuppose that because reductionism is a fruitful strategy for certain scientific problems that these means that a philosophical reductionism must be universally valid.

The Oxford Companion to Philosophy considers three aspects of philosophical reductionism: ontological, methodological, and theory.

Ontological reductionism "refers to the belief that the whole of reality consists of a minimal number of entities." For example, humans are really just self-organising biochemical systems or that the world is just a collections of quarks, leptons, and gauge fields. This often appears to involve value judgements as to what is "real" and what is not.

Methodological reductionism claims that ``the best scientific strategy is always to attempt explanation in terms of ever more minute entities.’’ One can differentiate this methodology further in terms of micro-reductionism and macro-reductionism. The former focuses on explaining phenomena at one strata in terms of the next lowest level strata. For example, genetics can be understood in terms of DNA. Macro-reductionism goes much further, claiming to explain phenomena at one level in terms of phenomena at a much lower strata. Socio-biology is an example of macro-reductionism. Micro-reductionism has proven to be an extremely successful and fruitful strategy; it has led to a simplification of ideas and unification of knowledge. However, it is contentious whether it is always the best scientific strategy. Although macro-reductionism has prominent, articulate, and passionate advocates I am unable to think of any specific cases where it has actually been able to produce knowledge that has been accepted by a majority of scientists in the associated field.

Theory (or epistemological) reduction considers how one theory which replaces a prior one reduces to it in an appropriate limit. For example, Einstein’s theory of special relativity reduces to classical Newtonian mechanics in the limit of objects moving much less than the speed of light. Similarily, the equations of quantum physics reduce to those of classical mechanics in the limit of large objects. This was advocated by the ``Unity of science’’ movement in the 1930’s and the influential work by Nagel [The Structure of Science, (Harcourt, New York, 1961)] who claimed that reduced theories were just logical consequences of the reducing theory. Thomas Kuhn, considers that theory reduction is not possible because new theories often involve concepts and modes of explanation that are ``incommensurate’’ with prior theories. For example, although in appropriate mathematical limits quantum theory reduces to classical mechanics, they do not agree on whether on can ascribe a definite position and momentum to a single particle. A less radical view emphasizes the complexity and subtleties associated with theory reduction because it requires limiting procedures, coarse graining and approximations. [R. Batterman, The Devil in the Details. Asymptotic Reasoning in Explanation, Reduction, and Emergence (Oxford: Oxford University Press, 2002).]