Most science fiction stories of time travel suppose that spacetime is subject to what cosmologists call “negative curvature”, which produces what are popularly known as “butterfly effects”. A typical time travel plot consists of a very well-meaning but somewhat less well-informed protagonist trying to correct some wrong in the past, only to find that his “correction” fails to leave everything else intact. On the contrary, it triggers an alternative history that increasingly deviates from the known history, often with hugely comic or tragic consequences, depending on the story's author. In the absence of an operable time machine, it is difficult to say whether time travel would actually wreak such havoc on human history, although most of today's gloom about climate change and the world's ecology more generally presumes the ubiquity of butterfly effects.

Nevertheless, it is clear that when it comes to concepts and institutions that mean a lot to us, we act as if they are relatively immune to the tinkerings of a time traveller. They are presumed to be grounded in something more substantial than the exact historical circumstances from which they arose, so that even if some bumbling visitor from the future managed retroactively to prevent Newton from publishing Principia Mathematica, someone else would have done so eventually and the history of science would return to a form that we recognize. As in the case of democracy, scholars can dispute whether science began in ancient Athens or seventeenth-century England, but whether the idea turns out to be twenty-four or a mere four centuries old, we still hold it in utmost esteem.