Scientists and the symmetry principle: an email exchange with Steven Weinberg

Earlier this week I received an email from Steven Weinberg, the physicist whose book To Explain the World was critically reviewed by Steven Shapin earlier this year. I commented on Shapin’s review in this essay, and in the process I attributed errors to Weinberg that he disclaimed in his email. Weinberg asked that I publish his email, and my reply, on this blog. I was happy to oblige. As I point out in my reply, Weinberg’s reaction to my essay is an additional reason to adopt the symmetry principle that I defended in earlier posts.

Here are the relevant parts of Weinberg’s email:

Dear Dr. Bycroft,

I found a good deal with which to agree in your February 24 blog essay ‘Why historians shouldn’t write off scientists: On Steven Shapin’s review of Steven Weinberg’s [To] Explain the World,’ which I have just seen. But I was infuriated by your reference to ‘the real error in Weinberg’s book,’ that I have ‘judged past scientists by first establishing whether their theories were true or false, and then by assuming that the true ones must be the result of sound reasoning and the false ones the result of incompetence.’

No one who had read my book with an open mind could possibly reach such a conclusion. Throughout my book I was concerned much less with whether scientific theories were right or wrong, than with whether their proponents were advancing the methodology of science. For instance, as noted by a reviewer in the latest issue of The New York Review of Books, I emphasized that the good guess of Democritus—that matter consists of atoms—did not show that his reasoning was sound, and that in fact he had offered no reasoning to support this guess. Like all the pre-Socratics, his theory had no bite—it led to nothing that could allow it to be confirmed. Also, I explained that, although Ptolemy was wrong about the solar system, he had made an important methodological contribution in adjusting the parameters of his theory to get quantitative agreement with observation. I was careful to point out the errors of Galileo about comets and of Newton about diffraction.

Later in your essay, you refer to other supposed ‘errors’ of mine. In short, of these too I am innocent.

To your credit, you were honest enough to admit that when you wrote this you had not read the book, and were relying on Steven Shapin’s description. The dangers of such a procedure are evident.

Here is my reply:

Dear Dr. Weinberg,

Thank you for taking the trouble of reading my blog essay on Steven Shapin's review of your book. I am glad that you found ‘a good deal with which to agree’ in the essay. I hope it was clear from the essay that my main target was Shapin’s review, not your book. If had intended to write a review of your book, I certainly would have read the book before casting aspersions on its content, as I do on two occasions in my essay.

On one occasion (‘As Shapin points out, it ignores...’) my phrasing is ambiguous. I seem to say that I have information about your book that is independent of Shapin's review. This is not the case, and I have inserted a footnote to this effect.

On the other occasion I say that I am making an educated guess based solely on Shapin’s review. Perhaps I should have added that this was a risky guess based on a small sample of citations plucked from the review. In any case, I have spelled this out in the footnote I just mentioned.

In making this educated guess I was less interested in evaluating your book than I was in drawing attention to what is known in the trade as the ‘symmetry principle’ or ‘methodological relativism.’ As I have argued elsewhere on my blog, this principle is badly misunderstood. Even professional historians of science have trouble saying what it is and why we should adhere to it. There is a tendency to confuse the principle with the idea that historians should never evaluate past science, or that they should never invoke the truth of a belief to explain that belief, or even that intellectual factors matter little in scientific debates. The result is that many legitimate and useful practices are criticised on the grounds that they violate a principle that few historians are able to articulate in a coherent manner. My remarks on your book were part of my effort to separate the many putative symmetry principles from the real one.

What is the real symmetry principle? I gave my version in the post in question. It is that we should not ‘judge past scientists by first establishing whether their theories were true or false, and then by assuming that the true ones must be the result of sound reasoning and the false ones the result of incompetence [or worse].’

You imply in your email that you agree with this principle. I find this very heartening. In the past, historians and sociologists have had great trouble persuading scientists that there is a version of the ‘symmetry principle’ that is worth taking seriously. The One Culture?, which I reviewed recently on my blog, and which featured chapters by yourself and Shapin, contains ample evidence of this problem. It looks like we have found a solution.

The symmetry principle would be worthless if no-one was ever tempted to violate it. I am grateful for the examples you invoke to show that you have not done so. I think the Galileo and Newton examples are equivocal. They would only be telling if you specified the causes of the errors in question. Did these men go wrong because they were temporarily disabled by a bad method or a social or psychological bias, or because even the best methods and the most clear-eyed scientists will go wrong some of the time? Both explanations are consistent with the symmetry principle, as I understand it. But the former is also consistent with a violation of the principle.

The Democritus and Ptolemy examples are more convincing. There are comments under my blog essay that defend your book along similar lines, eg. the comments by Michael Weiss (here and here) and by Will Thomas (here). As you point out, however, it is better to read a book than to take someone's word on its contents. So I will reserve judgement on your methodology until I have had a good look at To Explain the World.

I hope that the two offending passages in my essay do not obscure my broader point, which is that the methodological gap between professional historians (like Shapin) and scientist-historians (like yourself) is smaller than the former group sometimes makes out. The symmetry principle is one aspect of this. Another aspect is the fact that scientist-historians have actually done the thing (science) whose history they study. You may be interested in a more recent essay in which I look into this issue, starting with your statement that history is something that you ‘visit as a tourist.’

Weinberg wrapped things up with the following:

Dear Dr. Bycroft,

I’m heartened by the convergence in our views. In particular, I do agree with the ‘symmetry principle’ as you have stated it, though this is one symmetry principle that had previously escaped my attention. I agree that the comments in my note to you about Newton’s fudging and Galileo’s error are not relevant to this principle. I included these comments only to exonerate myself from accusations of hero worship. As you say, the relevant examples cited in my note were those having to do with Democritus and Ptolemy.

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