We know from reading Voltaire’s letters that he likes quoting – French literature in abundance, but also a fair amount of Latin. There is often a strong sense that he is quoting from memory, which is more than likely the lasting mark of his Jesuit teachers at Louis-le-Grand, who put Latin at the centre of the curriculum. Indeed, Voltaire had the benefit of some renowned Jesuit scholars as his teachers, notably Le Père Porée, who famously taught a ‘Senecan’ prose style, and Le Père Thoulier (later the abbé d’Olivet), a distinguished Cicero scholar who remained on friendly terms with Voltaire throughout his career.
Latin verse in particular, played a preponderant role in Voltaire’s education, as poets were at the heart of college teaching, and Virgil, Ovid, and Horace were by far the big three since the 16th century at least. The Jesuits taught primarily by way of daily recitals (recitatio) of verse required by all students: ‘On attachait à la recitatio une importance dont nous n’avons pas idée aujourd’hui…’ (Dainville, p.175). Thus, students at Louis-le-Grand all committed large chunks of Latin verse to memory as both a means of imitation for learning to write, and also as a method of retaining information, as Voltaire would elsewhere describe the pedagogical approach of the Jesuit Claude Buffier: ‘Il a fait servir les vers (je ne dis pas la poésie) à leur premier usage, qui était d’imprimer dans la mémoire des hommes les événements dont on voulait garder le souvenir’.
Given this background, we aimed to examine Voltaire’s use of Latin quotations across his massive collection of correspondence, described by Christiane Mervaud as ‘perhaps his greatest masterpiece’. The Besterman edition of Voltaire’s correspondence, originally published in some 50 print volumes, and digitised in the early 2000s as part of the Electronic Enlightenment project, contains 21,256 letters of which 15,414 are written by Voltaire himself. It is astonishing, then, that this masterpiece remains relatively unstudied. Besterman identifies Latin passages when they are from the major writers (Horace, Virgil, Ovid, Lucretius) – the authors for whom there were concordances easily available in the 1950s and 1960s. In the case of lesser poets like Manilius, however, Besterman was obliged to leave the passages unannotated. These passages can now be easily identified thanks to new methods developed in the digital humanities. In particular, as part of this year’s research programme in the Voltaire Lab, we compared all of Voltaire’s letters to Latin digital sources in an effort to systematically identify all of his Latin quotations, while at the same time, as we’ll see below, exploring the social and intellectual networks over which these quotations were exchanged.
Using sequence alignment algorithms designed to identify literary text re-use at scale –developed in collaboration with the ARTFL Project at the University of Chicago – we identified some 672 Latin citations in Voltaire’s correspondence by comparing the letters to the Packard Humanities Institute’s Classical Latin Texts (PHI) digital corpus. The PHI contains essentially all Latin literary texts written before A.D. 200, as well as some texts selected from later antiquity. The resulting alignments allow us to move beyond Besterman’s ad hoc manner of identifying quotations towards a more systematic understanding of Voltaire’s use of Latin authors.
After some data pruning – the inclusion of several commentators and grammarians from Late Antiquity in the PHI dataset meant that there were some repeated matches that were spurious – we reduced our set of Latin passages to 342 citations used by Voltaire himself to his various correspondents. Here is a list of these quotations by Latin author in descending order:
Overwhelmingly Voltaire prefers to quote Latin poets; and that Horace, Virgil and Ovid should be the top three is hardly surprising, though the presence of Horace is dominant. There is breadth as well as depth here, and the list goes beyond the usual suspects to include minor figures such as Manilius, Statius, and Cato the Elder. Does this mean, for instance, that Voltaire is quoting someone like Manilius from memory? If so, how interesting and altogether unexpected.
The next important question we broached was concerned with the recipients of Latin passages, i.e., who are the adressees of the letters in which these Latin quotations appear? In all we found 101 different recipients of at least some Latin, out of 1,465 total recipients in Voltaire’s correspondence (roughly 14.5 %). This is quite small, as a proportion of addressees overall. So how can we gloss these names as members of a group, or network of Latin quotations?
Using the ‘Procope’ social network ontology of the French Enlightenment, established by Dan Edelstein et al., at Stanford, we were able to automatically assign social categories to our list of addressees, which while not a perfect system, nonetheless helped us understand the fundamentally ‘elite’ status of this sub-set of Voltaire’s correspondents.
Gender is an obvious criterion that is apparently lacking: all addressees are male apart from one. Given that men learned Latin, and women didn’t, the use of Latin quotations is self-evidently gendered in this case. This is further reinforced by the manner in which Voltaire uses two verses by Virgil with La Duchesse de Choiseul, his one female addressee, in a letter from 1771:
‘Pour moi, Madame, qui les aime passionément je vous dirai
Ante leves ergo pascentur in æthere cervi
Quam nostro illius labatur pectore vultus.’
‘Vous entendez le latin, Madame, vous savez ce que celà veut dire:
Les cerfs iront paître dans l’air avant que j’oublie son visage.’ 
After quoting the two lines from the Bucolics, Voltaire goes on to translate them for Madame de Choiseul, even though she can presumably understand the Latin – a case of early-modern ‘mansplaining’ in action.
Within the group of 101 addressees, there is a clearly-defined social group of old, close friends from school (those with whom he had learned Latin), as well as an overlapping sub-group in Normandy, or in one case from Voltaire’s early law career:
Addressees from Louis-le-Grand, where Voltaire learned Latin:
- The Marquis d’Argenson (later foreign minister)
- The Comte d’Argenson (later war minister)
- The Duc de Richelieu (soldier and leading courtier)
- The Comte d’Argental, conseiller au parlement de Paris
- Pierre-Robert Le Cornier de Cideville, conseiller au parlement de Rouen
Other old friends from the overlapping Normandy/law group:
- Formont, a wealthy, talented light poet who was also friends with Cideville.
- Theriot, a an early friend of Voltaire’s, from when they were both young apprentice lawyers, who was also friends with Formont and Cideville.
Otherwise, we find many cultivated acquaintances in this list who are themselves authors: Frederick, Algarotti, D’Alembert, etc.; along with one of Voltaire’s teachers from Louis-le-Grand: d’Olivet, translator of Cicero and Desmosthenes into French, elected to the Académie in 1723. Clearly, Voltaire’s use of Latin was a means of determining readership. By constructing an epistolary community with selected groups of correspondents, Voltaire underscored their shared experiences and humanist culture.
But, to what extent was this sort of cultural exchange reciprocal? I.e., if Voltaire writes to you quoting Latin poets, do you feel obliged to respond in kind? What does it mean, for instance, that Voltaire uses Latin in so many letters to Frederick, and yet the prince never once uses Latin in return? Socially, the 41 respondents identified belong by-and-large to the same ‘elite’ categories of government or aristocracy, although there is a markedly greater presence of hommes de lettres (an ‘intellectual network’ that overlaps with the ‘social networks’ drawn from Procope) in this second list. See Table 3.
These are just some of the preliminary results we have begun to process in the context of a larger project on Voltaire’s culture of text re-use (including his penchant for ‘self-plagiarism’). As with most digital humanities projects, initial computational analyses don’t always produce ‘clean’ results, or cut-and-dried interpretations: some of the results have to be examined carefully, and some – as was the case for the grammarians and commentators mentioned above – will prove spurious or misleading. One begins asking one set of questions – can we identify Voltaire’s use of Latin and verify Besterman’s attributions – and end up with new ones: e.g., with whom did Voltaire use Latin, and how? Equally, we could extend these questions by examining other literary quotations, e.g., from French or Italian authors and by including other correspondence collections, comparing Diderot and Rousseau’s use of Latin, for instance, to that of Voltaire.
Ideally, this sort of experimental research approach also generates new research questions, ones that would have been difficult to frame outside of the digital environment. In this case, we were quickly confronted with the notion of what constitutes an instance of ‘re-use’ as opposed to an allusion or more oblique cultural reference. For example, our algorithm identified this passage from Cicero’s epistles:
‘Vale. CICERO BASILO S. Tibi gratulor, mihi gaudeo. te amo, tua tueor. a te amari et quid agas quidque agatur certior fieri volo…’
as a potential re-use employed by Voltaire in a letter to Marmontel from 1749:
‘Si vous recevez ma lettre ce soir, vous pourrez m’envoyer votre poulet pour m. de Richelieu, que je ferai partir sur le champ. Te amo, tua tueor, te diligo, te plurimum, &c.’ 
Is this re-use or not? Besterman makes no mention of Cicero in his annotation, but rather places this passage into a more generic class of ‘Roman epistolary formulas’. But perhaps there is more going on here; perhaps the model of Cicero’s epistles – central to the Jesuit syllabus – remains at the forefront of Voltaire’s mind when he himself is in the act of letter-writing. With the sorts of addressees for whom Voltaire uses Latin quotations he may likewise use a Ciceronian subscription. Here the Ciceronian model shapes Voltaire’s epistolary rhetoric.
Finally, pushing this line of enquiry a bit further, we came across another discovery: there are reduced versions of the passage, “Vale. Te amo”, which Voltaire uses extensively in the correspondence, and in particular with the social network of old school friends outlined above. This passage is in fact too small to be identified by our matching algorithms, and we would furthermore be a bit hard-pressed to classify it as a singularly Ciceronian borrowing. And yet…
– Nicholas Cronk and Glenn Roe
 See François de Dainville, L’Education des jésuites (XVIe-XVIIIe siècles) (Paris, Minuit, 1978).
 Voltaire, Siècle de Louis XIV, ‘Catalogue des écrivains’, OCV, vol.12.
 See Maria Teodora Comsa, Melanie Conroy, Dan Edelstein, Chloe Summers Edmondson, and Claude Willan, ‘The French Enlightenment Network’, The Journal of Modern History 88, no. 3 (September 2016): 495-534.
 [D17251]. Voltaire [François Marie Arouet], ‘Voltaire [François Marie Arouet] to Louise Honorine Crozat Du Châtel, duchesse de Choiseul [née Crozat]: Monday, 17 June 1771’. In Electronic Enlightenment Scholarly Edition of Correspondence, University of Oxford.
 [D3918]. Voltaire [François Marie Arouet], “Voltaire [François Marie Arouet] to Jean François Marmontel: Friday, 2 May 1749”, in Electronic Enlightenment Scholarly Edition of Correspondence, University of Oxford.