The passage shows Buridan to disagree with Aristotle on a pivotal point of Peripatetic cosmology in which there can be no beginning. Buridan's disagreement with Aristotle was anchored in the Christian belief in creation out of nothing and in time or "in the beginning," the conceivably most radical departure from pagan eternalism.And now, let us hear Duhem:
[see below for Duhem's quote of the passage]
In Buridan's account of the manner in which all motion had begun in the first beginning, Duhem rightly saw a clear anticipation of Newton's first law, or the law of inertial motion. Without saying much more, Duhem certainly brought into focus a most unexpected aspect of the unfathomable riches hidden in the phrase, "in the beginning," lurking behind Buridan's words, "when God created the world."
The ultimate significance of Buridan's statement was nothing less than that science could not have its beginning except as the fruit of the most fundamental of all beginnings. This certainly made sense from the viewpoint of theology, although most theologians still have to learn about Duhem's discovery, now almost a century old. From the scientific viewpoint, so often held to be antagonistic to theology, the insight of Buridan should at least conjure up a tremendous difference, the one between the circle (or wheel or swastika) and the arrow. The circle is a stamp on the scientific sterility of all great ancient cultures, all under the spell of the treadmill of eternal recurrences. The arrow, as a symbol of linearity, is expressive of a beginning which is a move in the forward direction. It is a beginning that put Western culture on the move and secured for it a pre-eminent global position.
[Jaki, Scientist and Catholic: Pierre Duhem]
This hypothesis of impetus, imparted to the projectile by the hand or by the machine that launched it, is seized upon by a physicist of genius, Jean Buridan. He takes it, toward the middle of the 14th century, for the foundation of a dynamics with which "all the phenomena are in accord."
The role which the impetus plays in this dynamics of Buridan is exactly that which Galileo will attribute to the impeto or momenta, Descartes to the quantity of motion, and, finally, Leibniz to the live force. So exact is that correspondence that in order to set forth in his Academic Lessons Galileo's dynamics, Torricelli often repeats the reasoning and almost the very words of Buridan.
This impetus, which would remain without change within the projectile if it were not constantly destroyed by the resistance of the medium and by the action of gravity contrary to the motion, this impetus, let us state it, Buridan takes, for equal velocities, for something proportional to the
contained within the body. He conceives and describes this quantity in terms almost identical with those which Newton will use to define mass. With equal mass, the impetus is as much greater as is the velocity. Wisely, Buridan abstains from specifying further the relation between the magnitude of impetus and that of the velocity. More daring, Galileo and Descartes grant that this relation reduces itself to proportionality. They thus obtain for impetus, for the , an erroneous evaluation which Leibniz will correct.
As does the resistance of the medium, gravity too continually diminishes, and finally annihilates the impetus of the projectile launched upward because such a motion is contrary to the natural tendency of gravity. But in an object that falls, the motion conforms to the tendency of gravity. Therefore the impetus too continues increasing and the velocity, during the motion, must constantly increase. Such is, according to Buridan, the explication of acceleration observed in the fall of a body, an acceleration which Aristotle's science already knew but of which the Hellenistic, Arabic, and Christian commentators of the Stagerite [Aristotle] gave unacceptable explanations.
This dynamics, as set forth by Buridan, presents in a purely qualitative, though always accurate, manner the truths which the notions of living force and of work allow us to formulate in quantitative language.
The philosopher from Bethune [John Buridan] is not the only one to profess this dynamics. His most brilliant disciples, Albert of Saxony and Nicole Oresme, make it known even to those who are not from the clergy.
When there is no resisting medium, when no natural tendency similar to gravity opposes the motion, the impetus keeps an invariable intensity. The projectile to which one imparted a translational or rotational motion continues moving indefinitely with an invariable velocity. It is in this form that the law of inertia arises in the mind of Buridan. It is in this form that it will be inherited by Galileo.
From this law of inertia Buridan draws a corollary whose novelty we must now admire.
If the celestial orbs move eternally with a constant velocity, they do so because, according to an axiom of Aristotle's dynamics, each of them is subject to an eternal mover of unchangeable power. The philosophy of the Stagerite requires that such a mover be an intellect separate from matter. The study of intellects moving the celestial orbs is not only the crowning of the peripatetic metaphysics; it is also the central doctrine around which turn all the neoplatonist metaphysics of the Hellenist and Arabic philosophers. The Scholastics of the 13th century do not hesitate to accept, in their Christian systems, this heritage of pagan theologies.
It is right here that Buridan has the audacity to write these lines: "At the creation of the world, God moved the heavens with motions identical to the ones with which they actually move. Therefore He then imparted to them impetuses by which they continue to move uniformly. Not encountering any resistance contrary to them, these impetuses are never diminished nor destroyed.... Within this view it is not necessary to posit the existence of intellects that move the celestial bodies in an appropriate manner."
This idea is stated by Buridan in various contexts. Albert of Saxony lectures on it and Nicole Oresme, in formulating it, finds this analogy: "Apart from the effort, this is altogether similar to the procedure when a man constructs a clock and lets it move itself."
If one wants to separate by an exact line the domain of ancient science from that of modern science, it has to be drawn, we believe, at the moment when John Buridan conceived that theory, at the very moment when one ceased to see the stars as if moved by divine beings, at the moment when one admitted that the celestial motions and the sublunary motions rested on the same mechanics.
[Duhem, Preface to Volume III of Etudes sur Léonard de Vinci quoted in Jaki, Scientist and Catholic: Pierre Duhem; my emphasis]
Being a Chestertonian I added a marginal note to the above quote by Jaki: "This contrast of circle/swastika versus arrow is reminiscent of Chesterton's contrast of the circle versus the cross in his The Ball and the Cross and Orthodoxy." Someday we shall explore this at length.
I shall add just one other excerpt - a very suggestive one for our Society:
Whether the individual Christian suspects this or not, his faith inoculates him with an anti-pantheistic serum. This is not so, however, in the case of the two other great monotheistic religions, Judaism and Islam. This is why Buridan's monotheism was different from Jewish and Muslim monotheism. It was a monotheism much stronger and was at work in Buridan's mind even he did not speak explicitly about it.Why do I say suggestive? Certainly this provides us with one of our future projects: to find these manuscripts and reprint them with critical analysis and commentary.
And the same is true of the leading intellectuals among Buridan's contemporaries and during the next ten or twelve generations, leading up to Newton. Nobody among Buridan's contemporaries took issue with his notion of impetus that was carried by his students of the Sorbonne to faraway universities in Europe. Manuscript copies of Buridan's commentaries on Aristotle's On the Heavens still can be found in Bologna, in Oxford, in Salamanca and in Cracow. It is from those manuscripts in Cracow that the young Copernicus learned about the doctrine of impetus which he used in the De Revolutionibus to answer objections to the earth's twofold motion in the heliocentric system. Or he may have found them in manuscript copies of the commentaries which Nicole Oresme, Buridan's most important student and eventually his successor at the Sorbonne, wrote on the same work of Aristotle.
[Jaki, "Christ and the History of Science" in A Late Awakening and Other Essays 576-7]