YES, HE CONTINUED, the astronomer repeated, now putting special stress on that word, or so it seemed to Leibniz, to see a great deal, a repetition and a stress in which Leibniz said he detected for the first time in their conversation a note of provocation: Prove that I cannot see what I claim to see! And, indeed, Leibniz’s inability thus far to prove or disprove the astronomer’s claim was already at this early stage starting to vex or even torment him, since the truth of it resided in a head less than one and a half feet from his own. The problem of entering a human head—a problem that hitherto had not even struck him as a problem at all, and which afterward would never strike him as a problem again, because before that day, and again after that day, but not on that day, Leibniz had absolute faith in the power of rational discourse to lay bare for us the contents of a human head—now struck him, only a few minutes into his exchange with the astronomer, as potentially insoluble, horrifically so. As the astronomer’s head talked, Leibniz half listened to it talk and half tried to figure out how to get inside it. It is rare, Leibniz noted, that we can pinpoint the precise geographical location of a truth, in this case a little less than a foot and a half away, in a quasi sphere (a head) whose circumference was itself a little more than a foot and a half, and yet have no idea what that truth actually is! He could cradle that head in his hands without getting an iota closer to the truth within it, and if, merely as a thought experiment, he were to reach over all of a sudden and crack that head open, the truth, “far from being freed from its shell like a walnut,” would simply perish along with the conditions that gave rise to it, i.e., the astronomer’s head, since we are dealing here with a contingent rather than a necessary truth, the-astronomer-without-eyes-can-see implying perhaps an absurdity but not a contradiction, as it hinges, Leibniz noted, on empirical rather than logical properties of eyes and sight.

This, incidentally, is the earliest appearance in the Leibniz corpus of the distinction between absurdity and contradiction, which in mature works like his Critical Remarks Concerning the General Part of Descartes’ Principles (1692) would serve to safeguard the possibility of the doubtful and the absurd, not to say the insane, from the Cartesian dictum that “what is doubtful should be considered false.”