The symmetry principle has been a central tenet of the history of science since at least the 1970s, and in my view it is a sound and valuable principle. However it is often confused with principles that are neither sound nor valuable, some of which are positively harmful for the study of past science. For example, the symmetry principle is sometimes expressed as the view that “truth” cannot explain the beliefs of past scientists. My main aim in this series so far has been to show that this view is hopelessly vague, and that on many readings it is false. In this post I will say the same about another aspect of the symmetry principle, namely the claim that historians should explain true and false beliefs “in the same way.” I’ll run through five readings of this claim, only the first of which deserves to be called the symmetry principle. Truth/falsity is not good evidence for good/bad reasons (Symmetry Principle) To use an example from my first post in this series, Galileo had a true theory of the moon and a false theory of comets. It is tempting to infer that he reached the moon theory through observation, experiment, and careful reasoning, whereas he reached the comet theory through arrogance, spite, and self-interest. That is, it is tempting to infer from the truth of the moon theory that Galileo held it for good reasons, and to infer from the falsity of the comet theory that he held it for bad reasons. Most historians of science, including me, think that both of these inferences are flawed. In general, the truth-value of a belief is not a good guide to the motivations of the believer. The inference from the former to the latter is what I call The Fallacy. Let’s not worry about why it is a fallacy, a question that I intend to answer in a later post. The important point is that it is a fallacy, and that the symmetry principle properly understood is simply the assertion that this is so. Hence I will call this assertion the Symmetry Principle (note the capital letters). Some might object to the Symmetry Principle on the grounds that there is no principled way of distinguishing good reasons from bad reasons. In practice, however, historians of science seem perfectly capable of making this distinction. The evidence for this is that they often criticise other historians for violating the symmetry principle, and to do this they must be to make the distinction in question. This is true no matter which version of the symmetry principle the critics have in mind, since all of the versions that I know of rely on some variant of the distinction between good and bad reasons. The Symmetry Principle must be distinguished from the following four claims. All of these claims have masqueraded as symmetry principles at one time or another. All of them look like the Symmetry Principle on the surface, but none of them follow from the Symmetry Principle and none of them should be called symmetry principles, either because they are false or because they have more to do with asymmetry than symmetry. Truth/falsity is never aligned with good/bad reasons (Exclusion Principle) The Symmetry Principle is quite a modest, minimalist principle. All it does is ban a certain inference, namely from truth-values to kind-of-reason. Banning an inference is not the same as saying that the conclusions of that inference are always false. Historians, just like scientists, can be right for the wrong reasons. Fallacious arguments can have true conclusions. For example: dead people cannot walk; Socrates cannot walk; therefore Socrates is dead. Or: my nicotine patch company will make more money if smoking causes lung cancer; therefore smoking causes lung cancer. Yet historians often interpret the symmetry principle to mean that truth-value is never aligned with type-of-reason. On this view, it is always an error to claim, of some true theory T, that a scientist believed T for good reasons and that his critics were mislead by their political interests or religious preferences. On this view, Galileo could not possibly have had good reasons for believing his moon theory but bad reasons for believing his comet theory. That option is ruled out a priori (hence my name for this principle, the Exclusion Principle). Galileo must have either had a mixture of reasons for believing both theories, or only bad reasons for believing both, or good reasons for the comet theory and bad reasons for the moon theory. Unlike the Symmetry Principle, the Exclusion Principle is hard to defend. Granted, there is a sense in which something like bad reasons are a precondition of all scientific activity. Social relations, for example, are sometimes associated with “bad reasons” for belief, yet it is impossible to do science worthy of the name without forming social relations of some kind. Here it is important to distinguish preconditions of belief from factors that shape belief, a distinction I borrow from Steven Shapin (although he might not agree with the application I am now making of the distinction). It is a precondition of scientific activity that someone pays for it (for example). But it does not follow that funding source always shapes belief, ie. that a scientist’s funding source is always a good explanation of why they believe theory X rather than a rival theory. It is also important to distinguish between absolute claims and relative claims. Probably there are no scientific beliefs that are pure in the sense that no bad reasons enter into their causal history. However it is very plausible that good reasons sometimes dominate over bad reasons. And there is no law of history that says that the good reasons never fall on the side of the true theories. Historians should never omit the good/bad reasons for false/true theories (Completeness Principle) The Symmetry Principle and the Exclusion Principle are claims about how historians reason about past science. By contrast, the third principle on my list is about what they should include in their books and articles. The Completeness Principle—the name is my own—states that whenever a historian discusses a true belief he must discuss the bad reasons the scientist had for holding that belief, and not just the good reasons. Conversely, discussions of false theories are lamentably incomplete if they omit good reasons in favour of bad ones. In other words, it is not enough for a historian to believe that truth/falsity is rarely aligned with good/bad reasons. He must also structure his narratives around this belief. It seems to me that the Completeness Principle is a bad idea, and it would be a bad idea even if the Exclusion Principle were true. The worry that lies behind the Completeness Principle is that books and articles might give misleadingly one-sided accounts of past science. There is something in this worry—after all, what’s the point of believing the Symmetry Principle if that belief is never reflected in our written work? Nevertheless, this worry should not lead to a blanket ban on narratives that are one-sided in the way I have just described. Such a ban would be like a ban on narratives that consider French science but exclude German science, or a ban on those that discuss botany but not mineralogy. When we read books that only cover French cases, or that only cover botany, the reasonable response is usually to assume that the author has focused on these cases out of convenience, or because that is what they know. We do not usually assume that the author denies the importance of German science, or of mineralogy. Likewise, if an author writes a book about Lavoisier’s biochemistry (for example) and says nothing about the French Revolution, we should not assume that the author denies the relevance of the French Revolution to Lavoisier’s biochemistry. Perhaps the French Revolution simply has little bearing on the argument the author is trying to make about Lavoisier’s biochemistry. Or perhaps the author omitted social and political factors in order to zoom in on Lavoisier’s attitude towards measurement. Historians should systematically omit good reasons in order to focus on the others (Methodological Relativism) Whereas the Completeness Principle says that historians should report good and bad reasons equally, this principle licenses them to omit good reasons. According to this principle, it is sometimes valuable for historians of science to ignore the experiments that scientists performed, the data they collected, or the chains of reasoning they articulated in order to defend one theory against another. These omissions are valuable, I take it, because they allow the historian to hone in on the social and political reasons for the beliefs that past scientists endorsed. I have no objection to Methodological Relativism as an occasional heuristic. Just as some authors may want to ignore the French Revolution in favour of Lavoisier’s experimental practice, others may want to do the reverse. However I agree with Will Thomas that Methodological Relativism is a recipe for disaster if it is read as a general rule of historical method. And this is how it tends to be read, in my experience, especially when it is conflated with the Symmetry Principle, which is indeed a general rule of historical method. Even if Methodological Relativism were a viable general principle, it would be misleading to call it a symmetry principle. It may be symmetric with respect to true and false theories, treating both in the same way, but it is Assymmetric with respect to good and bad reasons, since it focuses on the latter at the expense of the former. Bad reasons are the dominant causes of both true and false beliefs (Social Constructivism) This claim, like the previous one, is asymmetric with respect to good and bad reasons, since it gives priority to bad reasons. Unlike the previous claim, it does not just say that our narratives of past science should be written asymmetrically. Instead it says that that past science really was asymmetric. This claim accepts that good reasons were at work in past science, but insists that they were less powerful or less decisive or less fundamental than the bad reasons. I hesitate to include this claim on my list, since people do not usually call it a symmetry principle and because the phrase “social constructivism” is such a fraught one. I include the claim anyway because it looks a bit like some of the other claims on this list, and because readers might wonder where it fits into this survey. I’ve plumped for “social constructivism” as a label for this doctrine because, for all the ambiguity of the name, it is usually applied to the claim that one kind of cause (whether we call these “social factors” or “bad reasons” or something similar) makes a larger contribution to scientific beliefs than another kind of cause (“epistemic factors” or “good reasons” or something similar) . Another reason to mention Social Constructivism here is that it is often the end of a chain of reasoning that begins with the Symmetry Principle. For example, a common gambit is to observe that there are good reasons on both sides of many scientific debates, and to infer from this that good reasons cannot explain why people take the sides they do. After all, the argument goes, how can a commonality explain a difference? Needless to say, I do not endorse this argument or any of the others that lead from a laudable symmetry to a paralysing asymmetry. To describe these arguments would require another post, and this series is already too long for that. I’ll simply conclude that there are several different claims that have been called symmetry principles in the last few decades, that several of these claims are not worthy of that name, and that we should not confuse those imposters with what I have called the Symmetry Principle. In case you missed it, the Symmetry Principle is made up of the twin claims that the truth of a belief is not good evidence that the believer had good reasons for holding it, and that the falsity of a belief is not good evidence that the believer had bad reasons for holding it. The burden of my next post is to say why the Symmetry Principle is a good principle. This task is harder than it might seem, as we shall see. Expand post.