Writing on 16 December 1813 to “my dear friend and Baron,” Thomas Jefferson told Alexander von Humboldt that “America has a hemisphere to itself.” Thanks to isolation from Napoleon's war-torn Europe, the elder statesman Jefferson predicted that his younger friend Humboldt would live to see a future of peace and principle for “the American continent” (155). For his part, Humboldt, who had met Jefferson in 1804, during a brief five-week stop in the US at the end of his long expedition to the Americas, later referred to himself as “half an American” and “almost American” (“presque Américain moi-même”) (179–80). Although Humboldt never returned to the US, he and Jefferson maintained an ongoing relationship in letters written almost up to Jefferson's death in 1825.

An account of that relationship, Sandra Rebok's Humboldt and Jefferson: A Transatlantic Friendship of the Enlightenment, demonstrates the unexpected challenge of pairing two such famous figures. Despite the vast archive we have for each of these voluminously documented and self-documented figures (Humboldt's own record of his life's work remains unfinished despite the fact that it was so extensive and expensive as to almost bankrupt him), their relationship is most notable for what's missing. While Humboldt and Jefferson formed their famous celebrity friendship when they met in Washington during May and June of 1804, the final weeks of Humboldt's five-year expedition to the New World, that visit was not included in his Relation historique (Personal Narrative), the first three volumes of the published record of his 1799–1804 Voyage aux régions équinoxiales du nouveau continent (translated into English as Personal Narrative of Travels to the Equinoctial Regions of the New Continent), published between 1814 and 1829. His “30-volume voyage” covers Humboldt's travels as far as northern Colombia in May 1801, but not those in southern Colombia, Ecuador, Peru, Mexico or the United States.Footnote ¹ Scholars have long conjectured that either Humboldt never wrote the volume he planned to cover the period from mid-1801 to 1804 or, for some reason, he ordered it to be destroyed; the English translations are similarly incomplete.

So a major missing link turns out to be at the heart of Humboldt and Jefferson. Rebok acknowledges the evidentiary gap with an ultra-readable Appendix that includes the only extant account of the US visit by Humboldt himself, a short piece written on his way back to Europe, at the request of the American Philosophical Society, then translated into English and published as “Original Communication – Supplementary” in the Literary Magazine and American Register (1804). Not only does the title suggest yet another prior, missing piece, but the Appendix also includes all of the existing Humboldt–Jefferson correspondence from 1804 to 1825, eight letters from Humboldt (all written in French) and six (all in English) to him, the fullest record of their long friendship – but ironically or not, it's also incomplete (while Jefferson used a “polygraph” to copy all of his letters, Rebok notes that it's likely that some of the letters from the even more prolific Humboldt have been lost (54)).

While Rebok is thus scrupulous about noting the inaccuracies and omissions in the record, she does not grapple with the larger historiographical problem that there is only indirect evidence, from others, of the one and only time the two met, but rather keeps repeating it, returning to worry at it like a sore tooth. We are told about multiple missing pieces of evidence, from apparent errors in the record (Monticello as the supposed location of the Humboldt–Jefferson personal encounter is in doubt, with most sources mentioning Washington as their place of meeting), to a lost engraving of Humboldt that a friend of Jefferson's wanted to send to the President (a “very true-to-life engraved portrait of Mr. Humboldt that I thought would be a lasting reminder” (71)), to “strange omissions” in the topics of the day, especially that of slavery. This last is most critical, of course, reflecting the greatest possible sphere of conflict between the slave-owning Jefferson and the outspokenly anti-slavery Humboldt, the self-professed “half an American,” who explained that phrase in a letter to the New York Times shortly before his death in 1859: “that is, my aspirations are all with you; but I don't like the present position of your politics. The influence of Slavery is increasing, I fear. So too is the mistaken view of Negro inferiority” (37). Rather than systematically taking on the question of the attenuated Jefferson–Humboldt debate on slavery as well as on Native peoples, Rebok notes that “it seems strange that two such illustrious public intellectuals would fail to address it,” and comments, “perhaps if they had met at Monticello, it would have been more difficult to ignore the slavery question,” but concludes simply that “no record of any discussion remains” (61).

Here, again, is the comparative challenge of pairing two famous figures: what could there possibly be to fill in or speculate on with such a seemingly well-documented pair? It just so happens that another book on another Humboldtian twosome was published in relative proximity to Rebok's: Daniel Kehlmann's novel, Die Vermessung der Welt (2005), translated into English (by Carol Brown Janeway) as Measuring the World (2006), on Humboldt and the mathematician Carl Friedrich Gauss. The novel is a kind of alternative history, very different from Rebok's study, imaginatively counterfactual and speculative, addressing how Humboldt and Gauss did cross, and might have crossed, paths. Opening with their meeting in 1828, the novel flashes back and forward to their separate lives before and after, punctuated by eruptions of connection, where they meet mentally, and think of one another – Humboldt the geographer, experimentalist, traveler; Gauss the geometer, conceptualist, stay-at-home – as living parallel lines that converge, per Gauss's view contra Euclid. Not only do parallel lines meet but also space itself is “folded, bent, and extremely strange” (79–80). From this fantastic space–time-bending perspective, the parallel lifelines of these two nominally opposed figures come increasingly close together, until at the end the novel returns to their opening meeting point. “Humboldt imagined Gauss at that very moment staring through his telescope,” and “all of a sudden he could no longer have said which of them had traveled afar and which of them had always stayed at home” (252). Near the end, the novel even takes a moment to imagine the Humboldt–Jefferson encounter, which Kehlmann represents in less than two pages, mostly from the perspective of the dismissive Humboldt. Hosted by “a backwoods president” at a “pitiful gala dinner” in the city of Washington (“a building site,” everywhere under construction), located in “a tiny Protestant community on the edge of the world,” Humboldt speaks about “his joy at having left the sphere of an oppressive despotism” and “the nightmare of slavery,” at which point “their foreign minister, Madison,” kicks him under the table. “Jefferson had estates, whispered Madison. And? With everything that entailed” (180–82).

In contrast, Rebok puts her twosome together in a fairly flat, unimaginative comparison–contrast: her language is filled with “differences and similarities,” “parallels and discrepancies,” “despite the dissimilarities … certain aspects in common.” We can most likely trace back this uninspired approach, separating the figures via a mechanical compare–contrast structure within and across chapters, to the glaring lacuna at the center of their American encounter. She can't really put Humboldt and Jefferson together because they, her primary sources in both senses of the word, didn't do so. Rebok needs to take a different tack, more in tune with Humboldt himself, author of the notoriously unfinished thirty-volume voyage, a scientist at home with conjecture and hypothesis (not at all just a quantifier), an intellectual attracted to a kind of countertheorizing (about European knowledge of the Americas) that ends in probabilities and uncertainty. The more experimental perspective of a Humboldt might have better served her and her readers, authorizing her, for example, to speculate on the possible connections between the larger contradictions in the positions of Jefferson as slave-owner and master–father of Sally Hemings's children and Humboldt as the colonial traveler, civilizational thinker, and defender of slaves and indigenous peoples.

Finally, a Humboldtian analysis might have led to the way that the missing center of the US trip produced other seemingly unrelated anomalies. Humboldt was lionized as the “Baron” (by Jefferson and many other American contemporaries attracted to titles), a world-famous European intellectual gathering knowledge about America to refute the reigning Buffon–Raynal theory of the degeneracy of New World species. So much so that, as Rebok notes, more places are named for Humboldt in the US than anywhere else in the world, and in 1859 when he died there was a national outpouring that continued, and even grew, with later anniversaries throughout the nineteenth century. For the centennial of his birth in 1869, the New York Times dedicated the entire front page to him with a single-word headline: “Humboldt.” All this, yet Humboldt, as so many scholars repeatedly note, has now been largely forgotten in the US context, a relative unknown, himself become the missing center of that famous friendship, the parallel line as vanishing point.