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The notion of naturalness is important in particle physics, especially supersymmetry. I was a little surprised, then, that the idea, or at least the name, is apparently only ~30 years old ('t Hooft, 1980).

I know that Bayesian statistics, which formalises naturalness arguments with Bayesian evidence, is ~300 years old, and that with it one can elucidate connections between naturalness and Occam's razor (and even falsifiability). I don't know when those insights were first made, but surely not after 't Hooft?

Are there applications of the naturalness principle/fine-tuning in physics (or science) that significantly predate this 't Hooft definition? Or even Bayes? I should add that I consider naturalness to be distinct from Occam's razor.

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To me, it seems like there are 3 different concepts being discussed: (1) fine-tuning, (2) wanting unitless constants to be of order unity, and (3) wanting theories to have a simple form. The WP link defines "naturalness" as #2, although I don't think that's universally understood.

A very old example of #3 would be the history of models of the solar system, with, e.g., Kepler's laws being a lot simpler in form than epicycle models.

An example of #2 that is about 50 years old is that Brans and Dicke felt that the $\omega$ parameter in Brans-Dicke gravity should be of order unity. In their original paper describing their theory, they said prospectively that it should be of order unity, and implied that if it could be constrained to be much more than unity (which it soon was), they would consider that to be a falsification of the theory.

An example of #1 that dates back 44 years is the flatness problem in Big Bang cosmologies (WP says this dates to Dicke, 1969).

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  • $\begingroup$ I think the relationship between 1), 2) and 3) is cleared up with Bayesian statistics. I wouldn't consider epicycles - this seems to be an application of Occam's razor, rather than naturalness. i.e., the argument is similar, but Occam's razor more aesthetic than statistical, cf. naturalness. The flatness problem is an excellent answer, from reading Guth, I think it was realised, but not stated explicitly, that it was a statistical argument. I still hear lots of people say naturalness is purely aesthetic, which is frustrating. $\endgroup$
    – innisfree
    Commented Apr 28, 2013 at 19:16

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