Now, this astronomer had built, it was rumored, the longest telescope known to man, and therefore the most powerful, a telescope said to stretch nearly two hundred feet, reported Leibniz; but according to all known laws of optics the true power of a telescope is a function also of the power of the eye that peers into it, left or right, and the power of the eye is of course a function of the existence of the eye, and in this case neither eye existed, “neither the left nor the right.” Hence in mid-June when Leibniz set out from Leipzig for Bohemia—first by carriage, down sun-dappled forest lanes, then, upon reaching the foothills, by horse, past the black mouths of salt mines, and finally, in the high mountains, by foot, along muddy paths forged by goats, over passes still deep in snow, a journey it would be all but impossible to imagine him making in his big curly wig and expensive silk stockings if we did not have this textual evidence that he indeed made it, although the absurd image of the Leibnizian wig poking through a snowdrift, or of Leibniz himself stepping around a goat, has been sufficient in some quarters to cast doubt on the authenticity of the whole document—he knew that he was dealing most likely with a mystic, a madman, or a cunning fraud. But there was, as he explained to the Philosophical Transactions, a fourth and presumably final possibility, a possibility as intriguing as it was improbable: that he would encounter up there in the snowy mountains of Bohemia a man of reason, a man of science, whose prophesied flash of darkness would actually come to pass, who in other words stared up at the sky with his empty sockets and saw somehow what no other astronomer in the world could see, foresaw with no eyes what they could not foresee with two. If that were the case, Leibniz concluded, then when the light went on again four seconds later it would find the laws of optics in a shambles, knowledge in ruins, the human mind in an intimate embrace with the world, and the human eye in a state of disgrace.

He intended to reach the observatory by sunrise on the twenty-eighth, to stay there two days and two nights in order to “rigorously but surreptitiously assess the astronomer’s sanity” through “a series of subtly stringent interviews proceeding from the political to the theological,” i.e., from the lowest to the highest, “by way of the ethical, the logical, the astronomical, and the metaphysical,” and finally at noon on the third day, i.e., the thirtieth, to observe by the astronomer’s side the Moon’s predicted occultation of the Sun, the foretold four seconds of darkness on Earth, “the assessment sine qua non of his sanity.”

He could pass my assessment without passing God’s, Leibniz wrote.