Last Tuesday I was very pleased to be able to attend a research paper given at King’s by Dr Emma Spary (University of Cambridge) under the auspices of the Centre for the History of Science, Technology & Medicine, which is part of our Department of History. In her paper, entitled ‘Poisons and Providence in Old Regime Paris’, Dr Spary considered the ways in which making, controlling and writing about poisons in Paris between 1670 and 1789 created opportunities and crises for those who laid claim to scientific and medical expertise. Many of the writers she discussed were new to me – including Anne-Charles Lorry (1726–83), Achille-Guillaume Le Bègue de Presle (1735–1807) and Pierre Bulliard (1742–93) – though her discussion of Jean-Jacques Rousseau brought me back to more familiar territory, not least as earlier that day I had given a lecture on the Rêveries to first-year French students.
It was a thought-provoking paper and, for me, one of those papers that sends you away on the lookout for further examples. So my mind turned to Voltaire, who showed an interest in some of the high-profile cases of poisoning – or suspected poisoning – in the eighteenth century. Early in 1774, for instance, his attention was caught by the accusations then being levelled at the comic dramatist Beaumarchais, who was said to have poisoned his first two wives in order to get his hands on their money. Voltaire wrote to the comte d’Argental about the scandal on 17 January: ‘Caron [de Beaumarchais] is so delightful, […] so impulsive, so capricious and so witty that I’d go so far as to put my hand in the fire to convince you that he never poisoned his wives. Poisoners don’t make you burst out laughing; they’re usually chemists who are very serious and not very amusing’ .
As a freethinker Voltaire was also very interested in the metaphorical potential of poison, particularly in his fight against all forms of religious superstition and fanaticism. Writing to Frederick the Great on 20 December 1740, for instance, he explains that he does not share the optimistic view held by many that religious conflict is a thing of the past: ‘Those who say that the days of such crimes are over are, it seems to me, putting too much trust in human nature. The same poison still exists, albeit in a less developed form. This plague which seems to have been wiped out occasionally produces new shoots that are capable of infecting the whole earth’ . In the face of growing fanaticism, it is to Voltaire and his fellow freethinkers – the philosophes – that people must turn for the antidote, as he observes in a letter to d’Alembert – the co-editor, with Diderot, of the famous Encyclopédie – on 9 November 1764: ‘They alone have preached tolerance at a time when all the religious sects are being as intolerant as they can be. The freethinkers are the doctors of the very souls which the fanatics poison’ . And writing to both d’Alembert and Condorcet on 11 October 1770 about the virtue of freethinking – la philosophie – Voltaire notes: ‘When all is said and done, freethinking is the only consolation in life, while its opposite is what poisons life. Let things be, it is impossible to prevent people from thinking, and the more they think, the less unhappy they will be’ .
Voltaire’s confidence in the remedy of reasonable and reasoned behaviour finds perhaps its most memorable expression in his article on fanaticism in his famous Dictionnaire philosophique: ‘The only cure for this epidemic is the philosophical cast of mind which, as it spreads from one person to the next, eventually makes us gentler in our ways and prevents all attacks of this evil; because as soon as this evil starts making progress, we have to flee and wait for the air to clear. The law and religion are too weak a defence against the plague of souls; for these souls religion, far from being health-giving nourishment, turns into a poison in infected brains. These wretched people have continually in mind the example of Ehud, King Eglon’s assassin; of Judith, who slept with Holophernes and cut off his head; and of Samuel, who chopped King Agag into pieces: they fail to see that these examples, which are respectable in antiquity, are abominable in the present day; they justify their fury by invoking a religion that in fact condemns it’ .
Voltaire’s diagnosis – more’s the pity – stands the test of time. In Paris, Copenhagen and Sirte – to mention just a few of the most recent examples – fanatics justify their madness by invoking a religion that in fact condemns it. The challenge to freethinking and freedom of speech remains as poisonous as ever; the antidote needs cultivating now more than ever.
 Caron [de Beaumarchais] est si plaisant, […] si impétueux, si extravagant et si drôle, que je mettrais ma main au feu qu’il n’a jamais empoisonné ses femmes. Les empoisonneurs ne font point pouffer de rire; ce sont d’ordinaire des chimistes très sérieux, et très peu amusants.
 Ceux qui diront que les temps de ces crimes sont passés font ce me semble trop d’honneur à la nature humaine. Le même poison subsiste encore quoique moins développé. Cette peste qui semble étouffée reproduit de temps en temps des germes capables d’infecter la Terre.
 Eux seuls ont prêché la tolérance dans le temps que toutes les sectes sont intolérantes autant qu’elles le peuvent. Les philosophes sont les médecins des âmes dont les fanatiques sont les empoisonneurs.
 Au bout du compte elle est la consolatrice de la vie, et son contraire en est le poison. Laissez faire, il est impossible d’empêcher de penser et plus on pensera, moins les hommes seront malheureux.
 Il n’y a d’autre remède à cette maladie épidémique que l’esprit philosophique, qui répandu de proche en proche adoucit enfin les mœurs des hommes, et qui prévient les accès du mal; car dès que ce mal fait des progrès, il faut fuir, et attendre que l’air soit purifié. Les lois et la religion ne suffisent pas contre la peste des âmes; la religion loin d’être pour elles un aliment salutaire, se tourne en poison dans les cerveaux infectés. Ces misérables ont sans cesse présent à l’esprit l’exemple d’Aod, qui assassine le roi Eglon; de Judith, qui coupe la tête d’Holopherne en couchant avec lui; de Samuel qui hache en morceaux le roi Agag: ils ne voient pas que ces exemples qui sont respectables dans l’antiquité, sont abominables dans le temps présent; ils puisent leurs fureurs dans la religion même qui les condamne.