Occam's razor in science

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This wonderful essay on Occam's razor from the July 19, 2009 Wikipedia article[1] is preserved here for posterity. As it appears in Wikipedia it is templated as "unpublished synthesis," and will likely disappear in the future.

See also: Occam's razor

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In science, Occam’s razor is used as a Heuristic (rule of thumb) to guide scientists in the development of theoretical models rather than as an arbiter between published models.^[1][2] In Physics, Parsimony was an important Heuristic in the formulation of Special relativity by Albert Einstein^[3][4], the development and application of the Principle of least action by Pierre Louis Maupertuis and Leonhard Euler,^[5] and the development of Quantum mechanics by Louis de Broglie, Richard Feynman, and Julian Schwinger.^[2][6][7] In Chemistry, Occam’s razor is often an important Heuristic when developing a model of a Reaction mechanism.^[8][9] However, while it is useful as a heuristic in developing models of reaction mechanisms, it has been shown to fail as a criterion for selecting among published models.^[2]

In the scientific method, Occam's razor, or Parsimony, is an Epistemological, Metaphysical, or Heuristic preference, not an irrefutable principle of Logic, and certainly not a scientific result.^{[10][11][12][13]} As a logical principle, Occam's razor would demand that scientists accept the simplest possible theoretical explanation for existing data. However, science has shown repeatedly that future data often supports more complex theories than existing data. Science tends to prefer the simplest explanation that is consistent with the data available at a given time, but history shows that these simplest explanations often yield to complexities as new data become available.^[1][11]

When scientists use the idea of Parsimony, it only has meaning in a very specific context of inquiry. A number of background assumptions are required for Parsimony to connect with plausibility in a particular research problem. The reasonableness of Parsimony in one research context may have nothing to do with its reasonableness in another. It is a mistake to think that there is a single global principle that spans diverse subject matter.^[13]

As a methodological principle, the demand for simplicity suggested by Occam’s razor cannot be generally sustained. Occam’s razor cannot help toward a rational decision between competing explanations of the same empirical facts. One problem in formulating an explicit general principle is that complexity and simplicity are perspective notions whose meaning depends on the context of application and the user’s prior understanding. In the absence of an objective criterion for simplicity and complexity, Occam’s razor itself does not support an objective Epistemology.^[12]

The problem of deciding between competing explanations for empirical facts cannot be solved by formal tools. Simplicity principles can be useful heuristics in formulating hypotheses, but they do not make a contribution to the selection of theories. A theory that is compatible with one person’s world view will be considered simple, clear, logical, and evident, whereas what is contrary to that world view will quickly be rejected as an overly complex explanation with senseless additional hypotheses. Occam’s razor, in this way, becomes a “mirror of prejudice.”^[12]

It has been suggested that Occam’s razor is a widely accepted example of extraevidential consideration, even though it is entirely a metaphysical assumption. There is little empirical evidence that the world is actually simple or that simple accounts are more likely than complex ones to be true.^[14]

Most of the time, Occam’s razor is a conservative tool, cutting out crazy, complicated constructions and assuring that hypotheses are grounded in the science of the day, thus yielding ‘normal’ science: models of explanation and prediction. There are, however, notable exceptions where Occam’s razor turns a conservative scientist into a reluctant revolutionary. For example, Max Planck interpolated between the Wien and Jeans radiation laws used an Occam’s razor logic to formulate the quantum hypothesis, and even resisting that hypothesis as it became more obvious that it was correct.^[2]

However, on many occasions Occam's razor has stifled or delayed scientific progress.^[12] For example, appeals to simplicity were used to deny the phenomena of meteorites, ball lightning, continental drift, and reverse transcriptase. It originally rejected DNA as the carrier of genetic information in favor of proteins, since proteins provided the simpler explanation. Theories that reach far beyond the available data are rare, but General Relativity provides one example.

In hindsight, one can argue that it is simpler to consider DNA as the carrier of genetic information, because it uses a smaller number of building blocks (four nitrogenous bases). However, during the time that proteins were the favored genetic medium, it seemed like a more complex hypothesis to confer genetic information in DNA rather than proteins.

One can also argue (also in hindsight) for atomic building blocks for matter, because it provides a simpler explanation for the observed reversibility of both mixing and chemical reactions as simple separation and re-arrangements of the atomic building blocks. However, at the time, the Atomic theory was considered more complex because it inferred the existence of invisible particles which had not been directly detected. The stronger form of Occam’s razor favored by Ernst Mach gives rise to Logical positivism which rejected the Atomic theory of John Dalton for over 100 years, until the reality of atoms was more evident in Brownian motion, as explained by Albert Einstein.

In the same way, hindsight argues that postulating the Aether is more complex than transmission of light through a Vacuum. However, at the time, all known waves propagated through a physical medium, and it seemed simpler to postulate the existence of a medium rather than theorize about wave propagation without a medium. Likewise, Newton's idea of light particles seemed simpler than Young's idea of waves, so many favored it; however in this case, as it turned out, neither the wave- nor the particle-explanation alone suffices, since light behaves like waves as well as as like particles (Wave–particle duality).

Three axioms presupposed by the scientific method are realism (the existence of objective reality), the existence of observable natural laws, and the constancy of observable natural law. Rather than depend on provability of these axioms, science depends on the fact that they have not been objectively falsified. Occam’s razor and Parsimony support, but do not prove these general axioms of science. The general principle of science is that theories (or models) of natural law must be consistent with repeatable experimental observations. This ultimate arbiter (selection criterion) rests upon the axioms mentioned above. ^[11]

There are many examples where Occam’s razor would have picked the wrong theory given the available data. Simplicity principles are useful philosophical preferences for choosing a more likely theory from among several possibilities that are each consistent with available data. However, anyone invoking Occam’s razor to support a model should be aware that additional data may well falsify the model currently favored by Occam’s razor. One accurate observation of a white crow falsifies the theory that “all crows are black.” Likewise, a single instance of Occam’s razor picking a wrong theory falsifies the razor as a general principle^[11]. Note however that this only applies if the the razor is meant to pick the correct theory for all time; if this is not the case, and it is only applied to pick the simplest theory which fits all the currently known data and it is understood that, should new data arise, the razor will have to be reapplied, then the principle keeps its validity.

If multiple models of natural law make exactly the same testable predictions, they are equivalent and there is no need for parsimony to choose one that is preferred. For example, Newtonian, Hamiltonian, and Lagrangian classical mechanics are equivalent. Physicists have no interest in using Occam’s razor to say the other two are wrong. Likewise, there is no demand for simplicity principles arbitrate between wave and matrix formulations of quantum mechanics. Science often does not demand arbitration or selection criteria between models which make the same testable predictions.^[11]

[edit] References

1. ↑ ^{1.0 1.1} Hugh G. Gauch, Scientific Method in Practice, Cambridge University Press, 2003, ISBN 0521017084, 9780521017084
2. ↑ ^{2.0 2.1 2.2 2.3} Roald Hoffmann, Vladimir I. Minkin, Barry K. Carpenter, Ockham's Razor and Chemistry, HYLE--International Journal for Philosophy of Chemistry, Vol. 3, pp. 3-28, (1997).
3. ↑ Albert Einstein, Does the Inertia of a Body Depend Upon Its Energy Content? Albert Einstein, Annalen der Physik 18: 639–641, (1905).
4. ↑ L. Nash, The Nature of the Natural Sciences, Boston: Little, Brown (1963).
5. ↑ P.L.M. de Maupertuis, Mémoires de l'Académie Royale, 423 (1744).
6. ↑ L. de Broglie, Annales de Physique, 3/10, 22-128 (1925).
7. ↑ R.P. Feynman, R.B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. II, Addison-Wesley, Reading, (1964).
8. ↑ R.A. Jackson, Mechanism: An Introduction to the Study of Organic Reactions, Clarendon, Oxford, 1972.
9. ↑ B.K. Carpenter, Determination of Organic Reaction Mechanism, Wiley-Interscience, New York, 1984.
10. ↑ Alan Baker, Simplicity, Stanford Encyclopedia of Philosophy, (2004) http://plato.stanford.edu/entries/simplicity/
11. ↑ ^{11.0 11.1 11.2 11.3 11.4} Courtney A, Courtney M: Comments Regarding "On the Nature Of Science", Physics in Canada, Vol. 64, No. 3 (2008), p7-8.
12. ↑ ^{12.0 12.1 12.2 12.3} Dieter Gernert, Ockham's Razor and Its Improper Use, Journal of Scientific Exploration, Vol. 21, No. 1, pp. 135-140, (2007).
13. ↑ ^{13.0 13.1} Eliot Sober, Let’s Razor Occam’s Razor, p. 73-93, from Dudley Knowles (ed.) Explanation and Its Limits, Cambridge University Press (1994).
14. ↑ Science, 263, 641-646 (1994)