Last month I tried to show that historians can honour the symmetry principle without becoming skeptics about current scientific theories. We do not need to "forget" that the earth moves in order to see that there was once good reason to believe that it is stationary. But even if we do not need to forget this, perhaps we should try to forget it anyway just to be on the safe side? The aim of this short post is to explain why this kind of methodological relativism is not a good idea. Put simply, if we need to resort to this psychological trick in order to do good history then we have not understood the symmetry principle. My point is easily stated in the abstract. According to the symmetry principle, it is a fallacy to infer the quality of a person's reasons for holding a belief from the truth-value of their belief. In other words, the whole point of the principle is that truth-value--as judged by present-day science--tells us nothing about quality-of-reason. So if you are worried that your knowledge of present-day science will lead you astray in historical research, you have missed the point of the symmetry principle. Present-day science should not lead the historian astray because present-day science should not lead the historian anywhere. An analogy might help to get the point across. When small children are around we place the chocolate bars on a high shelf so that the children cannot reach them. The reason for this--or one of the reasons--is that small children do not grasp the fact that chocolate bars are bad for their health. If the chocolate was easy to access, children would eat it all at once and get sick. When adults are around, we place the chocolate bars wherever we like, because adults understand the link between overeating and illness. They can see chocolate, and even handle chocolate, without eating too much of it. The historian who cannot do good history in the knowledge of present-day science is like the child who cannot stay healthy within reach of chocolate. Present-day science can be dangerous in the hands of the historian, just as chocolate can be dangerous in the hands of a child. But these things are only dangerous if they are badly handled. To keep these things out of our reach is to concede that we cannot handle them properly. Once we know how to handle them properly, we should be able to store them in accessible parts of our kitchens (in the case of chocolate) or our minds (in the case of present-day science). But, you might object, it's not just children who need to store the chocolate on the top shelf or behind the muesli. Fully-grown adults can also give in to temptation, and surely they cannot be faulted if they use physical or psychological tricks to make up for the weakness of their will. After all, the important thing is to avoid getting sick, not to develop an iron will. We make use of this pragmatic principle every time we download software to block distracting websites or close the window to shut out the summery music being played outside our office. Why shouldn't historians of sciences help themselves to similar ruses? The answer is that historians who mishandle present-day science do not show a weakness of the will but a fundamental misunderstanding of the thing they are trying to understand, namely past science. The reason we should not infer quality-of-reason from truth-value is that there is such a thing as being right for the wrong reasons and wrong for the right reasons. These things are possible because the state of the evidence for a theory can change over time, because scientists can make lucky guesses on flimsy evidence, and for other equally significant reasons. If we are worried about violating the symmetry principle we should try to grasp these reasons rather than taking un-illuminating short-cuts such as forgetting or rejecting the theories of current science. Expand post.