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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| De motu antiquiora | 2/12 | https://en.wikipedia.org/wiki/De_motu_antiquiora | reference | science, encyclopedia | 2026-05-05T08:51:23.985272+00:00 | kb-cron |
== Influences and origins of Galileo's Pisan dynamics == Historians have debated the exact influences and development of Galileo’s early dynamics during his Pisan period and whether his early concepts of impressed forces were directly descended from the medieval impetus theory of the 14th century. Pierre Duhem proposed that Galileo’s Pisan dynamics was a continuation of the tradition taught by Jean Buridan and Nicole Oresme, in which Galileo continued to perfect the impetus theory throughout his career, potentially drawing influence from Giambattista Benedetti whose dynamics are generally described as a partisan of the impetus physics. Alexandre Koyré also noted Benedetti's potential influence on Galileo but argued, against Duhem, that Galileo’s Pisan dynamics was an attempt to achieve a coherent mathematical formulation of medieval impetus modeled after Archimedes’ statics, but instead of perfecting it, as suggested by Duhem, Galileo ultimately failed, which led him to abandon the medieval tradition and began anew into his Paduan period. Ernest Moody pointed out that Galileo’s arguments that rejected Aristotle’s explanation for projectile motion were exactly the same as those used by Buridan and Albert of Saxony, and was therefore not original; however, Moody argued against Duhem and Koyré that Galileo’s early dynamics were not modeled after the Buridan impetus, but rather based upon 11th-century Avenpace’s dynamics, which stemmed from the ideas of 6th-century John Philoponus of Alexandria. Moody suggests that it’s possible Galileo may have learned of Avenpace’s theories through Benedetti, but that it’s also possible that he may have learned of Avenpace through his contemporary, Jerome Borro, who wrote about Averroes’s criticism against Avempace’s arguments against Aristotelean physics. Further, Galileo was also influenced by his senior colleague at Pisa, Francisco Bonamico, who also discussed the problem of projectile motion in his own De Motu and mentioned that Philoponus is the originator of the theory of impressed forces. Moody suggests that Bonamico was acquainted with the medieval tradition of impetus physics, but only at a second- or third-hand account, especially in regard to the 14th-century contribution to mechanics, which is what led Koyré to assume that Bonamico’s views were an approximation of Buridan’s impetus and were the same as the impressed force theory of Philoponus, Peter John Olivi, and Francis of Marchia. Moody also credits the works of Benito Pereira, J. C. Scaliger, and Jacob Zabarella as potential influences.
== Distinguishing Galileo's early impressed forces from Buridanist impetus == Moody asserts that there is not only a developmental difference but also in meaning between Galilean impressed forces (virtus impressa) and Buridanist impetus: Buridan’s impetus was an “enduring reality” (res permanens) that would remain undiminished forever if left unimpeded by air and gravity, much like the modern treatment of momentum; whereas Galileo’s impressed forces were primarily self-depleting that is supplementally impeded by air resistance. This meant that the explanations for natural acceleration were radically different from each other since, for an object falling indefinitely in a void, the object’s Buridan impetus and speed would infinitely increase forever, whereas Galilean impressed force would eventually decay asymptotically to nothing as the object approaches a terminal velocity in the void. Another further distinction is made regarding rotational motion, specifically in explaining the rotation of the celestial spheres. Prior philosophers, such as Avicenna, al-Ghazali, Moses Maimonides, and most Christian scholastic philosophers, thought that the rotation of the celestial spheres required a constant force to maintain its rotation, and identified Aristotle's “intelligences” that pushed these spheres with the angels of revelation, thereby associating an angel with each of the spheres. Buridan proposed that impetus wouldn’t need to posit angels or “Intelligences” as movers of the heavens, for if we suppose that God, at the creation of the world, set the heavenly bodies in motion at their present rates of rotation, no further action by a “mover” would be required, because their original impetus would endure undiminished forever, in the absence of resistance or of opposed forces. Galileo, on the other hand, only partially answers this question by asserting that the rotation of the celestial sphere is not the result of a forced motion since the spheres do not recede from the center of the universe, nor is it natural motion since spheres do not appear to approach a natural place. This further raised the question if the rotation of the spheres moved perpetually or if they would eventually come to rest in the absence of a force, however, Galileo leaves the question unanswered.
== Synopsis of the essay portion == Galileo arranged his essay into unnumbered chapters; enumerated chapters were later added by Drabkin to facilitate cross-referencing. The reader is cautioned that, although Galileo’s arguments may appear sound throughout, some of his arguments contain errors due to flawed premises or mathematics.
=== Chapter 1: Heaviness and lightness ===
Galileo begins by defining heaviness and lightness, which is effectively the equivalent of the modern concept of specific gravity or relative density. Two substances are considered equally heavy when they are equal in both volume and weight. A first substance is called “heavier” than a second substance when the first weighs more than the second while both are equal in volume.
In this same arrangement, the second substance is called “lighter” than the first; however, Galileo defines "lightness (levius)" in the sense of being "less heavy (in gravitate minor)" than something else in a relative sense. In other words, all things have some degree of inherent heaviness and not an inherent lightness. In later reworkings of the De motu, Galileo emphasizes this distinction by removing the use of "light" throughout the work with substitutions of "less heavy." Thus, a reworked chapter 2 begins with, "That heavier bodies are by nature located nearer the center, and less heavy farther from the center, and why." With this distinction, Galileo is able to emphasize that upward motion is indeed forced motion by a heavier medium rather than by the object's inherent levity.