Kepler’s Theory of Force and His Medical Sources, Early Science and Medicine 19 (2014), 1-27.

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   1   ’                                        (  )  -    ©         ,  ,  |      .  /  -                  (  )  -  ISSN 1383-7427 (print version) ISSN 1573-3823 (online version) ESM* Université Paris 7 – CNRS, Laboratoire SPHERE UMR 7219, Case 7093, 5 rue Thomas Mann, F-75205 Paris CEDEX 13. The author wishes to thank Hiro Hirai (Radboud University Nijmegen), Giora Hon (University of Haifa), Justin Smith (Université Paris 7), Jean-Jacques Szczeciniarz (Université Paris 7), and this journal’s anonymous referees for their suggestions and comments. The Author would also like to thank Robert Westman (University of California, San Diego) for his comments on a very early version of this article. The Author is particularly grateful to Pietro Daniel Omodeo (Max Plank Institute for History of Science) for inviting him to present this paper to MPI’s Sphaera  workgroup, and to Karin Friedrich (University of Aberdeen) for her generous invitation to the University of Aberdeen’s special collections, where the idea for this article in its present form came together. Kepler’s Theory of Force and His Medical Sources  Jonathan Regier  Université de Paris 7 / UMR SPHERE (7219)  Abstract  Johannes Kepler (1571–1630) makes extensive use of souls and spiritus  in his natural philosophy. Recent studies have highlighted their importance in his accounts of celes-tial generation and astrology. In this study, I would like to address two pressing issues. The  rst is Kepler’s context. The biological side of his natural philosophy is not naively  Aristotelian. Instead, he is up to date with contemporary discussions in medically  a- vored natural philosophy. I will examine his relationship to Melanchthon’s anatomical-theological  Liber de anima  (1552) and to Jean Fernel’s very popular  Physiologia (1567), two Galenic sources with a noticeable impact on how he understands the functions of life. The other issue that will direct my article is force at a distance. Medical ideas deeply inform Kepler’s theories of light and solar force (  virtus motrix ). It will become clear that they are not a hindrance even to the hardcore of his celestial physics. Instead, he makes use of soul and spiritus  in order to develop a fully mathematized dynamics.  2                   (  )  -  Keywords  Johannes Kepler – Philip Melanchthon – Jean Fernel – Galen – force – causality – magnetism – species – spiritus  – spirit – innate heat – physics – cosmology – medicine – mathematics Introduction In the  Astronomia nova,  Johannes Kepler o   fered to the brave new world a Copernican physics, wherein the Earth’s motion could be explained in terms of an e  cient cause issuing from the sun. His physics is not a completely new production. It is usually depicted as a multiplication-of-species theory, adapted to  t with certain elements of William Gilbert’s (1544–1603) magnetic philoso-phy. The task of this article is to demonstrate that Kepler’s force at a distance owes a debt to sixteenth-century medical theories. An emphasis on the medical side of this force will also bring out the continuity in Kepler’s thinking, from his  rst book, the  Mysterium cosmographicum  (1596), to the  Astronomia nova (1609), after which his celestial physics does not change signi  cantly, although detail and depth are added. Historians have noticed the di   ference between these two accounts of force. In the  Mysterium , planetary motion results from the solar soul extending throughout space. In the  Astronomia , it results from a magnetic motive virtue (  virtus motrix ). As J.A.   Bennett has written, Kepler was in  uenced by Gilbert to make precisely this move from spiri-tual to physical forces. In the  New astronomy  the action of a soul in cos-mology is relegated to the rotation of the Sun, all other motions following by magnetic action according to the principles of Keplerian physics.   Bennett gives here what is still a mainstream interpretation, which seems cor-roborated by Kepler’s own words. At the end of his career, he writes this about the youthful  Mysterium : “If for the word ‘soul’ you substitute the word ‘force’,  you have the very same principle on which the Celestial Physics is established in the Commentaries on Mars , and elaborated in Book IV of the  Epitome of  Astronomy .”   But Bennett’s summary is wrong on one point and vague on   J.A.   Bennett, “Cosmology and the Magnetical Philosophy 1640–1680,”  Journal for the History of  Astronomy , 12 (1981), 165–177, 166.   Johannes Kepler, Gesammelte Werke [KGW], ed. Walther von Dyck et al. (Munich, 1937–), 8: 113. Johannes Kepler, The Secret of the Universe , trans. A.M. Duncan (New York, 1981), 203.   3   ’                                        (  )  -  another. It is wrong in saying that the solar soul is relegated to spinning the sun around; instead, the solar soul remains the source of force.   It is vague on its use of the term “spiritual force.” The word “spirit” ( spiritus ) is usually employed by Kepler either to signify the Holy Spirit ( Spiritus Sanctus ) or to signify one of the most concrete objects in Renaissance and early seventeenth-century natural philosophy: the medical spirits that travel through the body, serving as instruments of the soul. Readers of Kepler in translation can sometimes be caught unawares by the use of the English adjective “spiritual.” Aiton, Duncan and Field, in their translation of the   Harmonices mundi  , translate   facultas animalis  as “spiritual faculty” or idea ani-malis  as “spiritual idea.”   This is perfectly  ne, but it is important for readers to know that Kepler is referring to animal spirits here, that is, to an essential and everyday object of Galenic medicine. Kepler is not using spiritus  in a naïve way either. He would have been exposed to Galenic medicine during his education at Tübingen and to ‘best-selling’ sixteenth-century works of natural philosophy  written by physicians. While there are several studies on Kepler’s use of soul and spirit, both in his astrology and his accounts of celestial generation, I will consider these issues in relation to his dynamics. It is here where Kepler’s use of vital principles dovetails nicely with seventeenth-century discussions on substances and their causal powers. Existing studies have also been quite inter-nalist. Patrick Boner, for example, who has just published an excellent book on Kepler’s use of cosmological souls, does not treat Kepler’s relationship with his contemporaries on the nature of soul and spiritual instruments, nor is he inter-ested in the lively conversations on these topics among natural philosophers.      This has also been noted by Miguel A. Granada, “‘A Quo Moventur Planetae?’. Kepler et la question de l’agent du mouvement planétaire après la disparition des orbes solides,” Galilaeana: Journal of Galilean Studies,  7 (2010), 111–141, 136.   Johannes Kepler, The Harmony of the World  , trans. E.J.    Aiton, A.M.   Duncan, and J.V.   Field (Philadelphia, 1997). For ‘spiritual faculty’, see KGW, 6: 241. Kepler,  Harmony of the World,  327. For ‘spiritual idea’, see KGW, 6: 278. Kepler,  Harmony of the World,  374.   On Kepler’s use of souls in astrology and celestial generation, Patrick Boner has by far done the most work. See, for example, Patrick J Boner, “Kepler’s Living Cosmology: Bridging the Celestial and Terrestrial Realms,” Centaurus , 48, no. 1 (January 1, 2006), 32–39. Patrick J. Boner, “Life in the Liquid Fields: Kepler, Tycho and Gilbert on the Nature of the Heavens and Earth,”  History of Science , 46, no. 153 (2008), 275–297. For his recent book-length study see, Patrick J. Boner,  Kepler’s Cosmological Synthesis: Astrology, Mechanism and the Soul   (Leiden, 2013). For classic studies on Kepler’s astrology, which requires the presence of mathematical souls, see Sheila J. Rabin, “Kepler’s Attitude toward Pico and the Anti-Astrology Polemic,”  Renaissance Quarterly , 50 (1997), 750–770, and Judith V. Field, “A Lutheran Astrologer: Johannes Kepler,”  Archive for History of Exact Sciences , 31 (1984), 189–272.  4                   (  )  -  I would like to consider Kepler’s  virtus motrix  in relation to contemporary medical discussions, focusing on two of his main sources, Philip Melanchthon’s  Liber de anima  (1552) and Jean Fernel’s  Physiologia (1567). In the  rst part of this article, I will look at how medical notions of soul, spirit and vital heat inform Kepler’s early theory of solar force. In the second part, I will turn to Kepler’s mature theory of solar force: the species-theory that is in place from his optical  work (1603) onward. I will argue that species can be considered as ‘mathema-tized’ spirits. Disciplinary Considerations: Medicine and Astronomy  In the sixteenth century’s university setting, astronomy was subordinate to natural philosophy, from which astronomers were expected to take their  rst principles.   Kepler was deeply sensitive to astronomy’s low status and so felt that astronomy and physics should be uni  ed, forming a celestial physics (  physica coelestis ), which would provide the causes for celestial movement and mathematical schemes.   To this e   fect, he presented himself as a ‘philosophical’ astronomer. Humanist medical theory gave Kepler a way to think about causes, but there is more to the story than this. Medicine had a strong social and insti-tutional status. In the sixteenth-century context, it could serve as a hotbed of intellectual synthesis, in part because it could   push boundaries. Its hierarchi-cally elevated status was   xed in the structure of traditional universities, with the three professional faculties of theology, medicine and law located above the arts faculty. In other words, medical professors were higher placed on the insti-tutional ladder than professors of natural philosophy in the faculty of arts.   Professors of medicine were usually, albeit not always, also better paid, inside 6 On astronomy and disciplinary questions in the sixteenth century, see Robert Westman, “The Astronomer’s Role in the Sixteenth Century: A Preliminary Survey,”  History of Science , 18 (1980), 105–147. Michael H. Shank, “Regiomontanus on Ptolemy, Physical Orbs, and Astronomical Fictionalism: Goldsteinian Themes in the ‘Defense of Theon against George of Trebizond,’”  Perspectives on Science , 10 (2002), 179–207. Peter Barker and Bernard R. Goldstein, “Realism and Instrumentalism in Sixteenth Century Astronomy: A Reap-praisal,”  Perspectives on Science , 6 (1998), 232–258. Pierre Duhem, Sauver les apparences  (Paris, 2004). 7 His project is summed up in the full title of the  Astronomia Nova : the  New Astronomy  Based upon Causes, or Celestial Physics  (  Astronomia nova aitiologetos, seu physica coeles-tis ). Johannes Kepler,  New Astronomy , trans. William H. Donahue (Cambridge, 1992), 27. 8 See Charles B. Schmitt, “Aristotle among the Physicians,” in The Medical Renaissance of the Sixteenth Century , ed. Andrew Wear, Robert K. French, and Iain M. Lonie (Cambridge,   5   ’                                        (  )  -  the university and outside, where they could supplement their income through private practice.   In a particularly delicate moment of his young career, Kepler  was advised by the councilors of Graz to go into medicine.   Although he did not heed their advice, the sixteenth century knows many cases of mathemati-cians working their way up to a professorship in medicine, or at least to practic-ing medicine. Copernicus (1473–1543), among his many duties, was private physician to his uncle Lucas Watzenrode (1447–1512), the Bishop of Varmia. Jean Fernel (c. 1497–1558), passionate for mathematics and astronomy, was led into medicine for practical reasons, as John Henry has pointed out in a recent arti-cle.   Several well-known Lutherans also come to mind, like Georg Joachim Rheticus (1514–1574), Casper Peucer (1525–1602), and Duncan Liddel (1561–1613). Outside of the university, a professor of medicine or physician of repute could count the rich and powerful among his patients. Networking must account in part for the renown and success of natural philosophical works by successful physicians like Girolamo Cardano (1501–1576), Julius Caesar Scaliger (1484–1558), John Fernel, Petrus Severinus (1540/42–1602) and William Gilbert, a diverse lot that had at least two things in common: all of them did   count the rich and powerful among their patients and all of them, perhaps excluding Severinus, were read by Kepler.  1985), 1–15, at 4–5. For a more up-to-date treatment, see Paul F. Grendler, The Universities of the Italian Renaissance  (Baltimore, 2002), 267–269.9 I do not know of any study on professorial salaries in sixteenth-century Lutheran univer-sities, but I see no reason to doubt that the pay scale was similar to that in Italy, where, as Schmitt notes, junior medical professors were usually better paid than the most senior professors of natural philosophy. Schmitt, “Aristotle,” 274, n.   26. Grendler, by contrast, has pointed out that natural philosophers in Italian universities received a high salary because of natural philosophy’s importance as a preparation for medical studies: “The  rst ordi-nary professor of natural philosophy normally received the highest nonmedical salary in the faculty of arts. The exceptional natural philosopher enjoyed a salary equal to, and occasionally higher than, the best-paid professor of medicine.” Grendler, The Universities of the Italian Renaissance , 268.10 Nicholas Jardine, The Birth of the History and Philosophy of Science. Kepler’s “A Defence of Tycho against Ursus” with Essays on Its Provenance and Signi     cance  (Cambridge, 1984), 228.11 Also see John Henry, “‘Mathematics Made No Contribution to the Public Weal’: Why Jean Fernel (1497–1558) Became a Physician,” Centaurus , 53 (2011), 193–220.12 On Severinus and his relationship to Tycho Brahe, see Jole Shackelford, “Providence, Power, and Cosmic Causality in Early Modern Astronomy: The Case of Tycho Brahe and Petrus Severinus,” in Tycho Brahe and Prague: Crossroads of European Science , ed. John Robert Christianson, Alena Hadravovà, Petr Hadrava, and Martin Solc (Frankfurt, 2002),
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