Suppose that we happen to live in a universe in which magic is real, but only if everyone believes in it. It would follow that with the birth of the first scientist magic would shut down, and that scientist would be unable to find evidence of magic’s existence, other than in the accounts of those who had once experienced it first hand, which the scientist would of course attribute to delusion or hunger or psychotropic substance.
Of course, the consequences of magic would still exist in the world. If Merlin had indeed levitated that boulder and placed it over there, the boulder would still, of course, be over there. But the scientist would find other plausible explanations for the boulder’s location. The data supporting these other plausible explanations would be subject to omitted variable bias, with the omitted variable being, of course, magic. But the only way to test the influence of that variable on the boulder’s location would be to measure the historical incidence of magic. And that would be impossible because, again, in this universe as soon as even one person stops believing in magic, magic ceases there to be. There would be nothing to measure.
One might, of course, use proxy variables to try to measure historical magic—to measure historical magic by measuring its historical effects—but there would always be some other, non-magic variable that could be used to explain such effects, just as Ptolemaic geocentrism did a tolerably good job of predicting the planets’ positions in the sky despite being wrong. And, unlike magic, that variable would be measurable, and so would be more likely to convince science.
There being no way, therefore, to determine empirically whether we live in a universe in which there never was magic or in a universe in which there was magic but is none anymore, we are free to read the ancients, who tell us over and again about the interventions of the gods, as describing not delusion but a magic that really, actually was.
So, Greg Anderson, we need not merely pretend that what the ancients believed was real in order to understand them, we can also plausibly accept that the ancients were right.