Hobbes and the Galilean Law of Free Fall

  • Cees Leijenhorst
Part of the Boston Studies in the Philosophy of Science book series (BSPS, volume 239)


The Galilean science of motion was one of the major sources of inspiration of Thomas Hobbes’ mechanical natural philosophy. Already in 1634, at a time when his philosophical career had only just begun, we find Hobbes hunting — in vain — for a copy of the Dialogo sopra i due massimi sistemi del mondo (1632).1 Then, as a tutor of William Cavendish, the later Duke of Devonshire, Hobbes paid a visit to Galileo at Arcetri in 1635. There is an old legend that on this occasion Galileo inspired Hobbes to conceive a system of ethics more geometrico, which is one of those nice but most probably false myths of the history of philosophy.2 Due to his involvement with the Royalist cause in the English Civil war, Hobbes had to spend a long exile in Paris from 164o until 1651. Actually, this was the most productive period in his life. He was able to frequent on a daily basis the circle of Father Marin Mersenne, where Galileo’s new science of motion stood at the centre of attention. In the Mersenne Circle, Hobbes enjoyed the reputation of a good mathematician and interesting philosopher.


Free Fall Gravitational Attraction Heavy Body English Work Continuous Acceleration 
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  1. 1.
    Hobbes to William Cavendish, London 26 January / 5 February 1634: “My first businesse in London, was to seeke for Galileos dialogues [ ... ] it is not possible to get it for mony; There were but few brought ouer at first, and they that buy such bookes, are not such men as to part with them againe. I heare say it is called in, in Italy, as a booke that will do more hurt to their Religion then all the bookes have done of Luther and Calvin, such opposition they thinke is betweene their Religion, and naturall reason. I doubt not but the translation of it will here be publiquely embraced, and therefore wish extreamely that Dr. Webbe would hasten it;’ Hobbes, The Correspondence [Malcolm], I, p. 19Google Scholar
  2. 2.
    See Schuhmann, Hobbes. Une Chronique, pp. 46–47.Google Scholar
  3. 3.
    Mersenne was clearly shocked by some of the bold theses of the work, including the corporeality of the soul. As a consequence, he locked the manuscript away. It was only rediscovered in the twentieth century, still sometimes misleadingly referred to as Anti-White. On De motu, see Schuhmann, “Hobbes dans les publications de Mersenne:’Google Scholar
  4. 4.
    “Galilaeus, non modo nostri, sed omnium saeculorum philosophus maximus,“ Hobbes, Critique du De Mundo de Thomas White [ Jacquot e.a.] , p. 178. In the Examinatio et emendatio mathematicae hodiernae, Hobbes writes: “Primus qui scripsit de motu quod dignum lectu erat, fuit Galilaeus,” Hobbes, Opera Latina [Molesworth], Iv, p. 84.Google Scholar
  5. 5.
    “Galilaeus primus aperuit nobis Physicae universae portam primam, naturam motus,” Hobbes, De corpore [Schuhmann], p. 3. Here Hobbes paraphrases a statement made by Galileo in the 3rd Day of the Discorsi (Theorema II, Corollarius I De motu naturaliter accelerato): “[...1 the Author, who will have gone far to open the entrance and portal that has until now been closed to speculative minds;’ Galilei, Two New Sciences [Drake], p. 169.Google Scholar
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    On Descartes’ account of free fall, see Damerow e.a., Exploring the Limits, pp. 8–67.Google Scholar
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    On this “second Galileo affair;’ see also Galluzzi, “Gassendi e l’affaire Galilée:’ Google Scholar
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    On Hobbes’ relation to the Aristotelian tradition at this point, see Leijenhorst, The Mechanisation, pp. 172–179 and pp. 188–192.Google Scholar
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    See, for instance, Conimbricenses, In libros Physicorum, bk. 7, ch. 1, q. 1, art. 2, p. 224.Google Scholar
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  13. 13.
    Hobbes, Concerning Body (London, 1656), The English Works [Molesworth], I, p. 109. The latin original reads: ”Motus est continua unius loci relictio et alterius acquisitio;’ Hobbes, De corpore [ Schuhmann ] , p. 87 (= Opera Latina [Molesworth], I, p. 97).Google Scholar
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    ”Motor omnis movet vel pellendo vel trahendo,“ Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.], p. 180. See Aristotle, Physics, vII, 2, 243b16–17, and Gassendi, Epistolae duae de Motu impresso a motore translato: ”Caeterum, cum plures sint modi quibus causa externa movet, constat tarnen omnes ad duos tanquam praecipuas pertinere: impulsionem et attractionem. Age itaque experiamur, an-non motus rerum cadentium sive perpendicularis aliqua esse possit causa seu impellens seu attrahens seu potius impellens et attrahens simula’ Gassendi, Opera omnia, III, pp. 489b-490a.Google Scholar
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    “Supponamus igitur primum gravia deorsum pelli. Quern igitur gradum velocitatis pulsor gravi impertit ab initio, tali debet grave illud semper descendere, vel minore propter resistentiam aeris, nisi nova accedat impulsio iam descendenti. Quoniam ergo motus gravium descendentium continuo acceleratur, necesse est, si motus ille pulsio sit, ut pellens continuo incumbat praesensque sit cum eodem incremento velocitatis in omni parte aeris quem grave transit, quod imaginabile non est. Non est ergo unum aliquod pellens quod gravia perpetuo sequitur et impellit;’ Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.], p. 181.Google Scholar
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    Descartes to Mersenne, 29 January 1640: “Je viens de revoir mes Notes sur Galilée, où je n’ay veritablement pas dit que les cors qui descendent ne passent pas par tous les degrez de tardiveté; mais j’ay dit que cela ne se peut determiner sans sçavoir ce que c’est que la pesanteur, ce qui signifie la mesme. [ ... ] Et quand on frappe une boule avec un mail, je ne croy pas que vous pensiez que cette boule, au commencement qu’elle se meut, aille moins vite que le mail; ny enfin que tous les corps qui sont poussez par d’autres, manquent à se mouvoir, dès le premier moment, d’une vitesse proportionée à celle des cors qui les meuvent. Or est-il que, selon moy, la pesanteur n’est autre chose, sinon que les cors terrestres sont poussez reellement vers le centre de la Terre par la matiere subtile, d’où vous voyez aisément la conclusion,” Descartes, Oeuvres [Adam e.a.], III, pp. 9–10Google Scholar
  23. 23.
    “Caeterum, duo quaedam praemittenda sunt, quae inter alia bene multa magno Galileo debentur. Unum; corpus suopte decidens motu ea ratione accelerari, ut temporibus aequalibus maiora semper spatia pervadat, iuxta proportionem quam habent numeri impares inter se, initio sumpto ab unitate;’ Gassendi, Opera omnia, III, p. 483a. On Hobbes’ relation to Gassendi, see Schuhmann, “Hobbes und Gassendi.“Google Scholar
  24. 24.
    “Supponamus gravia deorsum trahi. Quod trahit igitur cohaerere fortiter debet cum ipso gravi quod trahitur, quod de aere cohaerente cum lapide durum est affirmare. Pratererea quod trahit, trahi ipsum quoque debet ab alio quod prius trahebatur, et sic continuo usque ad centrum terrae. Ibi vero si quid trahit, id faciet ascendendo, quod est contra naturam gravium;’ Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.],181.Google Scholar
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    On Kepler’s therory of gravitational attraction, see Overmann, Theories of Gravity, pp. 31–53. On Hobbes’ reaction to Kepler’s theory of magnetism and gravity, see Horstmann, “Ein Baustein;’ p. 147 and 154.Google Scholar
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    Hobbes, Concerning Body, The English Works [Molesworth], I, p. 434. The Latin original reads: “Virtutem autem magneticam illam sive attractionem et abactionem Terrae fieri arbitretur per species immateriatas. Hoc autem fieri non potest, propterea quod nihil movet nisi corpus motum et contiguum;“ Hobbes, De corpore [Schuhmann], p. 295 (= Opera Latina [Molesworth], I, p. 354).Google Scholar
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    Hobbes, Concerning Body, The English Works [Molesworth], I, p. 434. The Latin original reads: “Nam si hoc esset, non video, quare ovum ab ovo non attrahetur;” Hobbes, De corpore [Schuhmann], p. 296 (= Opera Latina [Molesworth], I, p. 354).Google Scholar
  28. 28.
    Gassendi, Opera omnia, III, pp. 521b and 526b. See Schuhmann, “Hobbes und Gassendi,” p. 163.Google Scholar
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    See Palmerino, “Infinite Degrees of Speed;’ p. 301.Google Scholar
  30. 30.
    “Neque sufficit dicere emitti ex magnete usque qualitatem quandam in ferrum, nisi dicantur simul emitti insensilia corpuscula quae sint quasi vehiculum eiuscemodi qualitatis. [ . . . ] Emitti ergo ex magnete corpuscula in ferrum debent quae agere in ipsum possint,“ Gassendi, Opera omnia, p. 492a.Google Scholar
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    Palmerino, “Infinite Degrees of Speed,” pp. 307–308.Google Scholar
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    The earliest known critique is that formulated by J.B. La Grange in his Principes de la Philosophie of 1675. See Palmerino, “Infinite Degrees of Speed;’ p. 301 n.82.Google Scholar
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    Freudenthal, “Clandestine Stoic Concepts,” p. 163. The same point is repeated by Osier, “How Mechanical was the Mechanical Philosophy?’” pp. 433–437Google Scholar
  34. 34.
    “Reliquum ergo est, ut tertio illo modo fiat [sc. descensus gravium], nempe ut sit motus quidam aeris ubique circularis, in quem si quod grave vi ascendat, motus ejus partium internarum, in quo motu consistit essentia ejus, conturbatur propter discordantiam et diversitatem viarum. Unde fit, ut grave semper se restituens perpetuo resiliat, donec perveniat ad centrum terrae, nisi vi prohibeatur;’ Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.], p. 181.Google Scholar
  35. 35.
    See Leijenhorst, The Mechanisation, pp. 163–165.Google Scholar
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    “Essentia, sive constitutio uniuscuiusque corporis specifica, id est ea per quam sensibus nostris dissimile apparet caeteris corporibus, in motu quodam consistit partium eius internarum;’ Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.], p. 289.Google Scholar
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    For examples, see Ariew e.a., “The Cartesian Destiny;’ pp. 316–32o.Google Scholar
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    Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.], p. 195.Google Scholar
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    The “cause of hardness and softness” is one of the many topics Hobbes and Descartes quarrelled about in their brief but vehement epistolary polemics organised by Mersenne in 1641. For Hobbes’ position, see his letter to Mersenne, Paris 28 january / 7 february 1641, Hobbes, The Correspondence [Malcolm], p. 62.Google Scholar
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    Hobbes adopts Kepler’s explanation of the diurnal motion of the earth, viewing the earth as a being that turns itself to the sun, thereby enhancing its “essential motion,” but turning away again so as to let its other parts enjoy the same beneficial effects. Though Hobbes rejects Kepler’s theory of “magnetical virtues,” he does not fully discard the pansensist and animistic elements of this explanation. On this issue, see Horstmann, “Ein Baustein;’ p. 144 and Leijenhorst, The Mechanisation, p. 99.Google Scholar
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    “Quae ego de hac re dicenda habeo, pauca sunt nec ad ipsam rei medullam pertinentia, sed solummodo praevia quaedam, quibus aliquanto propius ad eius cognitionem accedamus;’ Hobbes, Critique du De Mundo de Thomas White [Jacquot e.a.], p. 180.Google Scholar
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    See Hobbes, De corpore [Schuhmann], p. 343. The same theory in found in the Decameron Physiologicum, Hobbes, Opera Latina [Molesworth], Iv, p. 310.Google Scholar
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    Brandt, Thomas Hobbes’ Mechanical Conception, pp. 314ff. The following paragraphs summarise Leijenhorst, The Mechanisation, pp. 200–201.Google Scholar
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    Bernstein, “Conatus.“ See also Lasswitz, Geschichte der Atomistik, II, pp. 214–215.Google Scholar
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  • Cees Leijenhorst

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