Castel, Father (S. J.)

Edith Flamarion

1Louis Bertrand Castel, a Jesuit born in Montpellier on 11 November 1688, entered the Society as a novice in Toulouse on 16 October 1703 and remained in it until his death in Paris on 11 January 1757. In 1720, Fontenelle, secretary of the Académie des Sciences, took note of this brilliant mind after his first article in the Mémoires de Trévoux. On his recommendation and that of Tournemine, Castel was sent by his hierarchy to Paris. He immediately became a regent at the Collège de Louis le Grand and a journalist. He taught mainly mathematics (differential and integral calculus) and mechanics. Attentive to pedagogy and the applied sciences, in 1727 he published a brief Plan de mathématique abrégée, à l’usage et à la portée de tout le monde, principalement des jeunes seigneurs, des officiers, des ingénieurs, des physiciens et des artistes [‘Abbreviated plan of mathematics, intended for and within the capacity of everybody, in particular young lords, officers, engineers, physicists, and artists’]. Admitted in 1721 to the editorial committee of Mémoires de Trévoux, he played an essential role there, as collaborator and then organizer. In 1733, with the assistance of Brumoy, he reorganized the periodical. But in 1745 the arrival of Berthier as director of the journal led to his resignation. The hundred or so articles which he wrote, part of which appeared in the Mercure, dealt with the widest variety of subjects: “viscuous bodies”, “hard bodies”, an eclipse in China, shells, thunder, the invention of a hydraulic machine, morality, esthetics, politics, theology, etc. His reviews deal with the works of Newton, Huet, Fontenelle, Rameau, Bouillet, Réaumur… At the end of his life he had increasing difficulties with his church hierarchy: in 1741, Frogerais, the rector of Louis le Grand, asked him to recenter his work on religion and the struggle against deism: he devoted his last work to a refutation of Jean-Jacques Rousseau, L’Homme moral opposé à l’homme physique de Monsieur R*** , lettres philosophiques où l’on réfute le déisme du jour [‘The moral man opposed to the physical man of Monsieur R***: philosophical letters refuting modern deism’] (1756). After 1745) he sought outside support, in particular at the Académie des Sciences (in 1748 and 1752). His task as regent was lightened in 1750: he was chamber prefect for the students in physics. At his death, the homage in Mémoires de Trévoux was ambiguous: it conceded that the “philosophe geometer” had an “easy, rich, inventive mind”, but he was chided for being “constantly tempted by imagination”, which led him to “go beyond” sometimes “the line that geometry traced for him”.

2Castel wrote much, but he did not publish as much as he wished. Many texts are merely sketched out, others are lost or unpublished. He justified his constant interest in mathematics both morally (“no science less flatters the passions […], nor turns aside more evil”) and intellectually (“everything is clear, evident, proven in mathematics”). On the strength of his large 1728 work, a Mathématique universelle abrégée, which gives considerable space to geometry, he was admitted to the Royal Society of London. He was also a member of the academies of Rouen and Bordeaux (it is evidently Montesquieu who got him in, but without enthusiasm, and only in 1746: see Montesquieu correspondence, OC, t. XIX, letter 512). He wrote a Discours préliminaire à la tête de l’analyse des infiniment petits de M. Stône (1735) and a Géométrie universelle en dialogues (Amusements du cœur et de l’esprit, 1738). He treats marine sciences (Lettres […] sur la construction des vaisseaux, 1746) and the art of sieges (Mathématique appliquée: discours préliminaire au livre d’Azin sur la manière de défendre les places, 1731; Exercices sur la tactique, 1757).

3At the same time, he devoted himself to physics: “every function of our lives is an experiment […] everyone is a physicist […] and the only differences come from the accuracy of our thoughts”. What is “accuracy” of thought? Is it by following Descartes or Newton? Should one be a mechanist and support, with the Cartesians, the theory of “vortices of subtle matter” or ought one to accept the theory of gravitation urged by the Newtonians? Castel gives an account in the Mémoires of the “theory of vortices” (1721) and the “causes of gravity” according to Bouillet (1722). In 1724, he attempted to reconcile the principles of his faith and those of mechanism in the Traité de physique sur la pesanteur universelle du corps: even if gravity is the key to the system of the universe, freedom remains present: that of the Creator who “is not an automaton subjected to the mathematical laws of mechanics”, and that of man whose acts, free and natural, provoke a fruitful disequilibrium. The Cartesian Bouillet worries about “this odd mind”, and abbé Saint-Pierre approves it only with some nuances. The response of Castel then expresses a distance from “Cartesian hypotheses”, and in an exposé to Ramsay, he defends his system of gravity which goes “farther than that of M. Descartes but does not contradict it”; he reaffirms his faith in human freedom, which he will again express in his long review of Leibniz’s Theodicy (1737). But if Castel holds to claiming a Cartesian filiation, he nonetheless admires Newton: “Descartes had more ease and elevation. Newton, with less ease, was more profound.” (Le Vrai Système de physique générale de M. Isaac Newton exposé et mis en parallèle avec celui de M. Descartes, 1743).

4What was to confer celebrity on Castel was the extraordinary project that occupied him his whole life: the “ocular harpsichord” or “harpsichord for the eyes with the art of painting sounds”. In 1725, after collaborating with Rameau, he exposed in the Mémoires the proposal for an instrument which would “make sound visible”; he returned to it briefly in 1726 before publishing, in 1735, an exposé of nearly three hundred pages, which provoked a quarrel with Rameau. In 1737, Mairan refuted his theories in the Académie des Sciences, but Telemann reviewed them with interest in Germany, and Voltaire, in his Éléments de la philosophie de Newton (1738), praised the “ingenious philosophe”. When Castel in his Optique des couleurs (1740) questioned the analysis of Newton’s prism, the reactions were diverse and Voltaire mocked him (letter to Helvétius, 20 March). However, the Jesuit did not give up, and worked toward the realization of the instrument: dyed ribbons, mirrors, lamps, candles, “fans”, valves, he used everything to achieve it. From 1751 to 1753, he retraced the history of the enterprise in letters to Maillebois (Journal historique […] du clavecin en couleurs). But the end of his life finds him disabused. In 1753, the article devoted to the harpsichord in the Encyclopédie notes that the public regrets “that it is always being made and is not completed”.

5The “black Brahmin, very odd, half sensible, half mad” of Diderot’s Les Bijoux indiscrets (XIX) was the friend of Montesquieu until his death. So far we have no letter from Montesquieu to Castel, but we hear the Jesuit’s voice addressing open letters to him in the Mémoires de Trévoux and a dozen of his private letters to the Président have come down to us. In addition, Castel devoted to Montesquieu a not negligible portion of his 1756 work L’Homme moral opposé à l’homme physique de M***R. (Letters XVI to XIX and part of Letter XXI). He notes there that he was introduced to him soon after the publication of the Persian Letters by a “most noble and most virtuous lady” (Letter XVI), doubtless Mme de Pons. The two men came to be more and more closely connected. Castel confided to Montesquieu his projects and analyses, invited him to collaborate on the Mémoires when he sent him in 1724 Lafitau’s Mœurs des sauvages américains (OC, t. XVIII, letter 155, undated).

6In the Mémoires de Trévoux of June 1734, he gives a long extract of Romans, submitted to the author’s approval (OC, t. XIX, letter 388, [May 1734]); it was to him he addressed the long project of his “ocular harpsichord” in the Mémoires in August, September, October, November and December 1735 as well as in August 1739. And yet their relations underwent some change: Castel was not in on the publication of L’Esprit des lois. Disappointed, he obtained the work from its author only by dint of firm insistence. In the years 1749-1752, he offered his assistance to Montesquieu several times in vain, so he plays no role in the reviews in the Mémoires in April 1749 nor in February 1750. In an unpublished text (Plan d’impression, 1750?, conserved at the Bibliothèque royale de Bruxelles, published in Autour du P. Castel et du clavecin oculaire, p. 153-159), Castel, while asserting that Montesquieu “attacks the government without overly respecting religion, which he nevertheless treats gingerly because he fears it”, finds his theses difficult to oppose because “he is reason itself”. But in L’Homme moral he unambiguously pays homage to the Président: he evokes “the tender and intimate friendship” that binds them, notes that “as far as intelligence goes, M. de Montesquieu was an eagle” (Letter XVII) and asserts that “he was the fairest soul, the most candid, most truth-loving, especially with regard to religion” (Letter XVIII).

7Montesquieu for his part, at the beginning of their relations, had sufficient confidence in him to ask him, about 1723, to oversee the education of his son, Jean-Baptiste, placed at the collège Louis le Grand at age eight (letter of Castel, 7 August 1725, OC, t. XVIII, letter 143). A decade later, he called on him to reread Romans to draw his attention to passages that might cause problems (L’Homme moral, Letter XVII, and correspondence with Montesquieu, OC, t. XIX, letters 380 and 384). He took his opinions into account, although this did not prevent the work from suffering some cuts in order to obtain the privilège that permitted its distribution in France (Castel had thus clearly failed in his task), as well as his works and projects: he had acquired his Traité de physique sur la pesanteur universelle des corps of 1724 (Catalogue, no. [‣]). He noted the appropriateness of his analysis on evil (Spicilège, no. 555) but pointed out his errors (Spicilège, no. 347), found “rather poor” his treatise on the sublime in 1733 (Spicilège, no. 555) or mocked the Jesuit’s sophism on his “chromatic harmony” (Spic., no. 605). He judged with nuance the Vrai système de physique générale de M. Isaac Newton of 1743: the work is “clear” and the objections raised against Newton over the possible abuse of pure geometry were “good”; but he mocked the reproach of a tendency toward atheism by Castel who “like all Jesuits, does not fail to accuse M. Newton over the orthodoxy of his principles” (Spicilège, no. 565). If a certain distance opened up, the friendship was not finished: as death approached, it was Castel whom Montesquieu asked to be his confessor. Their friendship had lasted more than thirty years.



Review of Considérations sur les […] Romains in Mémoires de Trévoux, June 1734,

Reprinted in Montesquieu: mémoire de la critique, ed. Catherine Volpilhac-Auger, Paris: Presses universitaires de Paris-Sorbonne, 2003, p. 71-84.


Anne-Marie Chouillet, “Le clavecin oculaire du P. Castel”, Dix-Huitième siècle 8 (1976), p. 141-166.

Autour du P. Castel et du clavecin oculaire, Roland Mortier et Hervé Hasquin dir., Bruxelles: Éditions de l’université libre de Bruxelles, “Études sur le XVIIIe siècle” 23, 1995.

Jean Ehrard, “Une ‘amitié de trente ans’: Castel et Montesquieu”, in Autour du P. Castel et du clavecin oculaire, Roland Mortier and Hervé Hasquin dir., Brussels: Éditions de l’université de Bruxelles, 1995 (, reprinted in : Jean Ehrard, L’Esprit des mots. Montesquieu en lui-même et parmi les siens, Geneva: Droz, 1998, p. 69-81.

Bibliographical reference

Flamarion Edith , « Castel, Father (S. J.) », translated by Philip Stewart, dans Dictionnaire Montesquieu [online], directed by Catherine Volpilhac-Auger, ENS Lyon, September 2013. URL :