REVIEW ARTICLE

BURIDAN, OCKHAM, AQUINAS: SCIENCE IN THE MIDDLE AGES

William A. Wallace, O. P.

The Catholic University of America

Washington, D. C.

THE DEATH OF Ernest A. Moody in December of 1975 deprived the academic world of one of its foremost medievalists and intellectual historians, a person to be ranked surely with Pierre Duhem and Anneliese Maier for the many difficult texts he made available to scholars and for the novelty of the insights with which he continually stimulated them. Fortunate it was that just six months before his death the University of California Press saw fit to publish his collected papers, together with an autobiographical preface that explained his intellectual odyssey, why and when he wrote what he did from beginning to end, and how he finally evaluated the results of all his labors.¹ This series

¹ Ernest A. Moody, Studies in Medieval Philosophy, Science, and Logic. Collected Papers, 1933-1969. Berkeley: University of California Press, 1975. 477 pp., no index, $20.00. Apart from the Foreword and the Preface, the papers include: " William of Auvergne and His Treatise De Anima," pp. 1-110, written as Moody's M. A. thesis at Columbia in 1933 and previously unpublished; " John Buridan on the Habitability of the Earth," pp. 111-126, which originally appeared in Speculum, Vol. 16 (1941); " Ockham, Buridan, and Nicholas of Autrecourt," pp. 127-160, reprinted from Franciscan Studies, Vol. 7 (1947) ; " Ockham and Aegidius of Rome," pp. 161-188, also from Franciscan Studies, Vol. 9 (1949); "Laws of Motion in Medieval Physics," pp. 189-202, reprinted from The Scientific Monthly, Vol. 72 (1951); "Galileo and Avempace: The Dynamics of the Leaning Tower Experiment," pp. 203-286, reprinted from the Journal of the History of Ideas, Vol. 12 (1931); "Empiricism and Metaphysics in Medieval Philosophy," pp. 287-304, reprinted from The Philosophical Review, Vol. 67 (1958); "The Age of Analysis," pp. 305-320, reprinted from Proceedings and Addresses of the American Philosophical Association, Vol. 36 (1963) ; "A Quodlibetal Question of Robert Holkot, O. P., on the Problem of the Objects of Knowledge and Belief," pp. 321-352, reprinted from Speculum, Vol. 39 (1964); "Buridan and the Dilemma of Nominalism," pp. 353-370, which appeared in The Harry A. Wolfson Jubilee Volume, published at Jerusalem in 1965 by the American Academy for Jewish Research; " The Medieval Contribution to Logic," pp. 371-392, reprinted from Studium Generale (Heidelberg) , Vol. 19 (1966) ; " Galileo and His Precursors," pp. 393-408, which appeared in Galileo Reappraised, ed. C. L. Golino, Berkeley: University of California Press, 1966; " William of Ockham," pp. 409-440, reprinted from the Encyclopedia of Philosophy, Vol. 8 (1967); and "Jean Buridan," reprinted from the Dictionary of Scientific Biography, Vol. 2 (1970).

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of papers, together with Moody's three books,² stand as a monument to the man's impressive scholarship; they also afford those of us who knew and admired his work the opportunity to reflect on his achievement and to offer our own critique of his central theses.

My research interests have paralleled Moody's in a remarkable way, although we came to approach our common area from diametrically opposite directions. In my case Thomism provided the initial framework for my deep interest in Aristotle and in the medieval commentaries on the Physics, De caelo, etc., that led, by howsoever circuitous a route, to Galileo and his nuova scienza. In Moody's case it was Ockham who provided a similar inspiration, and this, oddly enough, precisely because of opposition to him from the Thomist camp. As he tells us,

What attracted me to Ockham, in the first instance, was the bad publicity given to him by the Thomists and particularly by Gilson, who portrayed him as a diabolical genius who tore down the beautiful edifice of scholastic philosophy and theology erected by Saint Thomas Aquinas. Since it was natural for me to side with the underdog, I felt the urge to find out what Ockham had to say.³

This enticed Moody into his doctoral study of Ockham's logic, which in turn led, after years of maturation, to his most famous work, Truth· and Consequence in Medieval Logic. Logic and methodology then gave way to concern with physical science, and here Moody found in Jean Buridan a congenial figure with whom to continue his Ockhamist interests. The fourteenth century became his focal point for ever more detailed studies, and the more he studied it, the more he saw that century as the one to which our own age is most in debt. As Lynn White points out in his foreword, quoting Moody's overall conclusion,

. . . if the later fourteenth century " has seemed to the historians of philosophy an age of decline, to the historians of science and logic it has seemed an age of rebirth and advance. . . . For better or worse, it gave a new character and direction to all later philosophy, of which we have not yet seen the end." ⁴

² Moody's books include The Logic of William of Ockham (New York and London: 1935), The Medieval Science of Weights (Scientia de ponderibus), coauthored with Marshall Clagett (Madison, Wisconsin: 1952), and Truth and Consequence in Medieval Logic (Amsterdam: 1953) .

³ Preface, p. xi.

⁴ Foreword, p. viii; see also pp. 300, 302.

My first contact with Moody came, predictably, shortly after the appearance of his classic essay, " Galileo and Avempace: The Mechanics of the Leaning Tower Experiment," ⁵ at which time I took issue with the mechanical doctrines he there attributed to St. Thomas and particularly with his attaching the labels " Cartesian " and " Platonist " to Aquinas's thought.⁶ As a result of an, initial interchange both of us prepared notes for the Journal of the History of Ideas and corresponded about them over a considerable period; in the end, however, neither was pleased that he had understood and met the other's objections, and by mutual consent we withdrew our manuscripts. Neither of us returned to the precise matter of the interchange, although I later attempted to set the record straight on Aquinas's contribution to medieval mechanics in my treatment of him in the Dictionary of Scientific Biography, without, however, making reference to Moody's interpretation.⁷ Fortunately, in the intervening years my Dominican confrere, James A. Weisheipl, has written two scholarly articles wherein he makes essentially the same points I had indicated to Moody, without himself being aware of that interchange.⁸ Since Aquinas's teaching is thus now well exposed in the literature, a few comments may serve here to relate that teaching to Moody's exposition of it in " Galileo and Avempace."

Paralleling his work in medieval logic, where he was able to translate the discursive Latin texts of the fourteenth century into the symbolic expressions of twentieth-century logic, Moody attempted to formulate a key problem of medieval and early modern dynamics in terms of equations that would be intelligible to twentieth-century physicists. He thus pictured the difference between Aristotle and Galileo over the possibility of motion through a void, a topic discussed in Galileo's Pisan work De motu, as captured in the two equations, V = P/M (Aristotle) and V = P -- M (Galileo), where V stands for the velocity or speed of motion, P for the motive power urging the body moved, and M for the resistive medium through which the body passes.⁹ In a void, of course, since there

⁵ Pp. 203-286.

⁶ P. 244.

⁷ " Saint Thomas Aquinas," Dictionary of Scientific Biography, Vol. 1 (1970), pp. 196-200, esp. p. 198.

⁸ " The Principle Omne quod movetur ab alio movetur in Medieval Physics," Isis, 56 (1965), pp. 26-45, and "Motion in a Void: Aquinas and Averroes," in St. Thomas Aquinas Commemorative Studies 1874-1974, 2 vols., Toronto: Pontifical Institute of Mediaeval Studies, 1974, Vol. 1, pp. 469-488.

⁹ Moody, Studies, p. 215.

is no resistance to motion, M takes the value of zero. For Aristotle this has the consequence that the motion becomes instantaneous, which is another way of saying that motion in a void is impossible; for Galileo, on the other hand, motion takes a definite value determined by the motive power alone, and thus motion through a void is possible. Then, searching out the medieval antecedents of these very different conceptions, Moody discovered them quite unexpectedly in the teachings of Averroës and Avempace: Averroës upheld the validity of Aristotle's equation, V = P/M, whereas Avempace rejected Aristotle's equation and in its place substituted the equation later to be found in Galileo, V = P -- M.¹⁰ More than that, Avempace's progressive views were not unappreciated in the Latin West; although some scholastics rejected them, " the outstanding defender of Avempace's theory was St. Thomas Aquinas," ¹¹ who not only defended that theory but actually adopted "Avempace's ' law of difference ' represented by the formula V = P -- M . . ."¹² Thus Aquinas, acting as an intermediary for Avempace, played a key role in the development of Galileo's new science.

Flattering as it may be to propose Aquinas as such a precursor of Galileo, Moody's way of doing so does not do justice either to Aquinas's discussion of the possibility of motion through a void or to Aquinas's exegesis of Aristotle's text. As Weisheipl makes clear, Aquinas did not subscribe to the view that the dynamic formula V = P/M represents Aristotle's own teaching, for he regarded the arguments in Aristotle's text on which this formula is based as merely dialectical and not in any way demonstrative.¹³ Thus Aquinas had no reason to endorse either that formula or an alternate one such as Avempace's. It is true that fourteenth-century thinkers, following Thomas Bradwardine, became interested in dynamic formulas of various types, and that earlier Averroës (whose views on this matter Aquinas regarded as omnino frivola¹⁴) had championed V = P/M as Aristotle's authentic teaching. But Averroës did this because of his idiosyncratic philosophical understanding of the principle omne quod movetur ab alio movetur and how that principle could be justified in the case of falling bodies. In no event did Aquinas agree with Averroës on such matters, although unfortunately Anneliese Maier thought that all scholastics

¹⁰ Ibid., p. 227.

¹¹ Ibid., p. 236.

¹² Ibid., p. 242.

¹³ Weisheipl, " Motion in a Void," pp. 476, 487.

¹⁴ Ibid., p. 480.

shared common views both on the principle omne quod movetur and on the problem of motion through a void--views that in her estimation constituted a fatal barrier to the rise of classical physics.¹⁵ Weisheipl has been at pains, because of Maier's widespread influence, to show how diverse were the teachings of scholastics on these matters, and particularly how nuanced was Aquinas's view, being incapable of ready assimilation into what is fast becoming a standardized exposition among historians of medieval science.¹⁶

Weisheipl's studies are mentioned here as only a mild corrective to some of Moody's statements in the " Galileo and Avempace " article, for Moody rightly discerned Aquinas's rejection of the more obvious aspects of Averroës's teaching, and this was indeed a contribution at the time of his writing. Since that time twenty five years have elapsed, and my own recent researches, mainly in Galileo's early notebooks, have uncovered further connections between Aquinas and Galileo.¹⁷ With regard to Moody's overall thesis these new discoveries work two ways: they serve to ground in an unsuspected fashion Moody's suspicion of Aquinas as an influence on Galileo, and at the same time they tend to diminish Ockham's importance and to highlight Buridan's--not indeed as an Ockhamist, as Moody thought, but as an unlikely transmitter of Aquinas's methodological doctrines to Galileo.

Buridan's importance lies in his explanation of the methodology of ex suppositione reasoning, a topic touched on in one of Moody's papers in this collection entitled " Ockham, Buridan, and Nicholas of Autrecourt." ¹⁸ In view of Buridan's well-known condemnation, while rector of the University of Paris, of Nicholas's teaching and the suspicion that this condemnation was actually directed against Ockhamism, Moody decided to study the complex relationships between Ockham, Buridan, and Nicholas to ascertain the precise target of the condemnation and whether Ockhamism was de facto involved. Moody's conclusion, which comes as no surprise, is that the condemnation was indeed against Nicholas but that it was not anti-Ockhamist, at least not against the type of Ockhamism advocated by either Ockham or Buridan.¹⁹ Moody points to various

¹⁵ Ibid., pp. 469-470.

¹⁶ Ibid., pp. 487-488.

¹⁷ The beginnings of these researches are reported in my article entitled " Galileo and the Thomists," which was published in the St. Thomas Aquinas Commemorative Studies (see note 8), Vol. 2, pp. 293-330; some of these results will have to be revised in light of my later studies reported in notes 26 and 29 below.

¹⁸ Moody, Studies, pp. 127-160.

¹⁹ Ibid., pp. 157-160.

passages in Buridan's commentaries on Aristotle where he defends, apparently against Nicholas, the validity of causal analysis and man's ability to achieve certain knowledge of nature; such passages, of course, can easily serve to align Buridan with the Thomistic tradition, as Moody was well aware.²⁰ What is surprising is Moody's attempt to align Buridan with Ockham's position on similar matters. Now Ockham's denial of local motion as a distinct reality and his clear assertion of the inapplicability of causal analysis to this phenomena was certainly not accepted by Buridan; had it been, the impetus theory would never have been developed, to say nothing of the subsequent studies in medieval dynamics that make Buridan and his followers so important for the history of science generally.²¹ And in the matter of certain, scientific knowledge of the world of nature, Buridan's commitment was much stronger than Ockham's; if it is to be identified with any medieval tradition, it fits more readily with Aquinas's than with that of the Venerable Inceptor.

Ockham, like Aristotle, had a theory of demonstration, but as De Rijk has made clear, for Ockham a demonstration is nothing more than a disguised hypothetical argument and thus is not completely apodictic.²² Unfortunately Moody reads Buridan with precisely this Ockhamist bias, and so he interprets Buridan's claim that scientia naturalis is capable of attaining truth and certitude in a rather peculiar way. Failing to understand, as I see it, the technique of demonstration ex suppositione, which for Aquinas could lead to true and certain results, Moody interprets Buridan's use of the expression ex suppositione to mean that Buridan is advocating a type of hypothetico-deductive reasoning as proper to the natural sciences. So he draws the inference that, with Buridan,

an ineradicable element of hypothesis is introduced into the science of nature, and, as its counterpart, the principle that all scientific hypotheses require empirical verification, and retain an element of probability which cannot be completely eliminated.²³

I do not believe that this is the correct meaning of Buridan's thesis. Its exposition occurs in Bk. 2, q. 1, of Buridan's commentary

²⁰ Ibid., p. 154.

²¹ I have given some references to these teachings in my Causality and Scientific Explanation, 2 vols., Ann Arbor: The University of Michigan Press, 1972-1974, Vol. 1, pp. 53-55, 104-109.

²² L. M. De Rijk, " The Development of Suppositio naturalis in Mediaeval Logic," Vivarium, 11 (1973), pp. 43-79, esp. p. 54.

²³ Moody, Studies, p. 156.

on Aristotle's Metaphysics, which inquires " Whether it is possible for us to comprehend the truth concerning things? "²⁴ Buridan answers the question affirmatively through a precise and thorough analysis of the types of evidence on which truth and certitude must ultimately rest. From this he draws an inference that is quite different from the one Moody attributes to him. Buridan's own words read:

It follows as a corollary that some people do great harm when they attempt to destroy the natural and moral sciences because of the fact that in many of their principles and conclusions there is no evidence simpliciter, and so they can be falsified through cases that are supernaturally possible; for evidence simpliciter is not required for such sciences, since it suffices for them that they have evidence secundum quid or ex suppositione. Thus Aristotle speaks well in the second [book] when he says that mathematical certitude is not to be sought in every science. And since it is now apparent that firmness of truth and firmness of assent are possible for us in all the aforementioned modes, we can conclude with regard to our question that the comprehension of truth with certitude is possible for us. ²⁵

To affirm that " the comprehension of truth with certitude is possible for us " seems to me to be quite different from affirming, as Moody does, that " all scientific hypotheses require empirical verification and retain an element of probability which cannot be completely eliminated." The latter affirmation would reduce science to dialectics, it would clearly eliminate apodictic certitude from all scientific conclusions, and this is precisely the error Buridan has set himself to refute.

Now it seems to me more than coincidental that Galileo made many epistemological claims for science and demonstration in the

²⁴ lohannes Buridanus, In metaphysicen Aristotelis quaestiones. . . , Paris: 1518 (reprinted Frankfurt a. M.: 1964), fol. 8r, Utrum de rebus sit nobis possibilis comprehensio veritatis.

²⁵ Ibid. fol. 9r: Ideo conclusum est correlarie quod aliqui valde mali dicunt volentes interimere scientias naturales et morales eo quod in pluribus earum principiis et conclusionibus non est evidentia simplex sed possunt falsiflcari per casus supernaturaliter possibiles, quia non requiritur ad tales scientias evidentia simpliciter sed sufficiunt predicte evidentie secundum quid sive ex suppositione. Ideo bene dicit Aristoteles secundo huius quod non in omnibus scientiis mathematica acribologia est expetenda. Et quia iam apparuit quod omnibus predictis modis firmitas veritatis et firmitas assensus sunt nobis possibiles, ideo concludendum est quid querebatur, scilicet, quod nobis est possibilis comprehensio veritatis cum certitudine. . . .

matters with which he worked, and that he, like Aquinas and Buridan, very frequently justified his results by an appeal to reasoning ex suppositione.²⁶ This technique, as I have explained elsewhere, is implicit in Aristotle's Physics and Posterior Analytics, and it was explicitly shown by Aquinas to be capable of generating strict demonstration in the contingent subject matters with which natural science is concerned.²⁷ However, most commentators on Galileo, and most translators of his works, fail to grasp the nuances of this methodology and interpret Galileo, as Moody interprets Buridan, to be advocating and employing the hypothetico-deductive methods used in twentieth-century science. Such methods, of course, could never achieve the results that Galileo claimed, either by demonstrating the truth of the Copernican system or by establishing the nuova scienza of local motion of which he was so justly proud. To see Galileo as practicing a method that derives from Aquinas, on the other hand, and perhaps via Buridan but surely not via Ockham, would be to locate him in a methodological tradition that provided adequate canons for attaining the demonstrative certitude he claimed, however defective he himself might have been in applying such canons to the materials he had at hand.

Moody, moreover, notes of Galileo that his medieval thought context was essentially that of the thirteenth century, and suggests that " the sources of [his] dynamics . . . are to be sought elsewhere than in the tradition of fourteenth century Oxford or Paris, or than in the tradition of fifteenth century Padua." ²⁸ Now my recent work on the sources of Galileo's Pisan notebooks, oddly enough, would appear to confirm the validity of this particular insight. Much yet remains to be done, for the work is still actively in progress, but results to date strongly suggest that the main source of Galileo's early writings on logic and the physical sciences were contemporary Jesuit professors at the Collegio Romano.²⁹ These

²⁶ The specific texts are discussed and analyzed at length in my article, " Galileo and Reasoning Ex Suppositione: The Methodology of the Two New Sciences," in Boston Studies in the Philosophy of Science Vol. 32, Proceedings of the Philosophy of Science Association 1974, eds. A. C. Michalos and R. S. Cohen, Dordrecht, Holland: D. Reidel Publishing Co., 1976, pp. 73-98 (currently in page proof).

²⁷ For a summary account see my "Aquinas on the Temporal Relation Between Cause and Effect," Review of Metaphysics, 27 (1974), pp. 569-584, esp. pp. 572-574.

²⁸ Moody, Studies, p. 274.

²⁹ I have reported these results in December 1975 at the annual conventions of the American Philosophical Association in New York and of the History of Science Society in Atlanta. The more significant conclusions are recorded in my article, " Galileo Galilei and the Doctores Parisienses," forthcoming in New Perspectives on Galileo, eds. R. E. Butts and J. C. Pitt, a projected volume in the University of Western Ontario Series published by D. Reidel & Co. (probably 1977),

Jesuits were all thoroughly trained in the Thomistic tradition, but they were also eager to search through and evaluate the common teachings of the Schools, and their works are replete with references to Averroists, Scotists, nominalists, and others. I would not be surprised if Galileo derived his knowledge of Avempace, for example, from the writings (mainly reportationes of lectures) of such Jesuits. And this fact alone would serve to explain why Galileo's discussion continues to focus on issues that were central in thirteenth-century thought, even though they touch tangentially on problems dating from the fourteenth and fifteenth centuries that have been regarded for so long as the seed bed of modern science. Moody's heroes, by his own admission, were the fourteenth-century thinkers who contributed much to logic and to the mathematical modes of thought that have become popular among philosophers in our own " age of analysis." Like many of us, he did his history of philosophy, of logic, and of science with an ulterior goal in mind: he thought that careful studies of the type he engaged in would cast light on present-day problems and perhaps point the way to new directions for the future.³⁰ Having such a goal did not corrupt his historical scholarship: withal he was careful, objective, dogged in his search for truth, and ever willing to pursue that search wherever it might lead. His loss at this time, needless to say, will be deeply felt, all the more because of the new research materials that are now becoming available on Galileo and his relationships to medieval science. That particular problem engaged much of Moody's effort over a long period of his life, and he was uniquely endowed to give a critical evaluation of the many factors that bear on its solution. My own reaction to the new materials (again, predictably) is that they connect Galileo's nuova scienza much more strongly with the via antiqua of Aquinas than they do with the via moderna of Ockham. This is not to say, of course, that Ockham and the nominalist movement were unimportant either for Galileo or for the rise of modern science. To be convinced of that all one need do is read these collected papers that summarize Moody's life work so well, and that now stand as such a fitting memorial to his scholarly endeavors.³¹

³⁰ Moody, Studies, pp. 287-304 and 305-320.

³¹ The research on which portions of this paper are based has been supported by the National Science Foundation (Grant No. SOC 7S-14615), whose assistance is gratefully acknowledged.