> I am always skeptical of retrospective claims of obviousness (partly from the number of times I have seen it made in math textbooks.) I agree with you about

> I am always skeptical of retrospective claims of obviousness (partly from the number of times I have seen it made in math textbooks.) I agree with you about your claim, but not about your justification. I believe that "historically obvious" things ("that's the first thing I would have thought of in that situation; how did it take them so long?") are usually subject to considerable ex post facto bias. On the other hand, when a mathematics textbook says that something is obvious, it means, or should mean: we've specifically set up the presentation to this point so that there's a unique best way to assemble the material so far, and that unique best way will accomplish the next step. This isn't always true when it's claimed, but it is possible in textbooks in a way that it isn't (or that has only a small probability) in history, since textbooks are consciously organised and history isn't.