De motu antiquioraGiusti, Enrico, "Galileo's De motu antiquiora," Nuncius 13, no. 2 (1998), 427–60. ("The Older Writings on Motion"), or simply De Motu, is Galileo Galilei's early written work on motion (not to be confused with Newton's De motu corporum in gyrum, which shares the abbreviated name, De Motu). It was written largely between 1589 and 1592,Stillman Drake, Essays on Galileo and the History and Philosophy of Science, Volume 1, University of Toronto Press, 1999, p. 216. but was not published in full until 1890. De Motu is known for expressing Galileo's ideas on motion during his Pisan period prior to transferring to Padua.
Galileo left the manuscript unfinished and unpublished in his lifetime due to several uncertainties in his understanding and his mathematics. It is unclear whether this book was initially made out to be a book in the form of a dialogue or a more conventional way of writing. The reason for this is that Galileo worked on this book for many years, creating multiple copies of each section. In the last parts of his work, the writing style changes from an essay to a dialogue between two people who strongly uphold his views. Galileo would later incorporate select arguments and examples from his De Motu into his subsequent works Le Mecaniche (On Mechanics), Discorso intorno alle cose che stanno in su l'acqua (Discourse on Floating Bodies), and Discorsi e dimostrazioni matematiche intorno a due nuove scienze (Discourses and Mathematical Demonstrations Relating to Two New Sciences).
Throughout De Motu, Galileo rejects Aristotle's views on the physics of motion, often with mocking tones, through various reductio ad absurdum arguments that demonstrate how Aristotle's assumptions on motion logically result in absurd conclusions that were contrary to observation or against his original assumptions, thus proving that the assumptions must be false. However, despite his frequent stinging criticism of Aristotle’s physics, Galileo’s De Motu still clung to the classical elements as a foundational cause for motion in which all matter moves toward its respective natural place in the universe.
He further proposes an alternative theory to motion in which, instead of motion being propagated by the rushing of air (as was taught by the Peripatetics), it is believed that the true weight of a body can only be measured in a void, that the weight of the body in a medium is modified by its buoyancy in the medium (i.e., apparent weight), that the weight resulting from this buoyancy causes the body's natural motion, that projectile motion (distinct from natural motion) is believed to be the result of an impressed forced that modifies a weight of the projectile, and that the impressed force depletes over time much like how a hot object returns to its natural coldness.
De Motu is notable for containing the earliest reference of Galileo’s interest in pendulums in which he observes that heavier objects would continue to oscillate for a greater amount of time than lighter objects. However, he misattributes this phenomenon as evidence that the impressed force in a moving body self-depletes faster in lighter bodies than in heavier bodies as opposed to air resistance having a greater effect on the lighter body.
It’s questionable how much of Galileo’s ideas in De Motu were original. Some of the ideas of the De Motu are found in antiquity, others in the Middle Ages and among Galileo’s immediate predecessors in Italy. The subjects discussed in the essay are largely the subjects that had long been under discussion in academic circles, but while the solutions put forth by Galileo to individual problems are not, in general, original discoveries, the work as a whole gives a distinct impression of originality. This is due to the underlying unity of conception, the skillful linking of ideas, the constant recourse to mathematics, and the lucidity of the reasoning and the style.
Publication history De Motu may have been originally intended for publication, but Galileo eventually abandoned it in an incomplete form. What remains now includes a first draft essay on motion, several reworked portions of the essay, a dialogue, a set of topics and propositions, and a series of fragmentary thoughts, notes, and memoranda.
Portions of the manuscript were first published in 1854 in Volume 11 of Eugenio Albèri’s edition of Galileo’s works, and the remaining portions were published in 1883 by Antonio Favaro. The manuscript was later published under the title De Motu in volume I of the National Edition of the works of Galileo, edited in 1890 by Favaro, which contained certain manuscripts written in Latin in Galileo’s own hand. The first and only English translation of the essay portion of De Motu translated by I. E. Drabkin was published in 1960 under the title On Motion and On Mechanics, which was included alongside Stillman Drake's translation of Galileo's Le Mecaniche.
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 Aristotelian 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.
Chapter 2: Heavy substances are by nature located in a lower place and light substances in a higher place
Galileo assumes a quasi-Aristotelian arrangement of the universe based on the classical elements where things move according to their proper place: earth rests at the center, water is above earth, air is above water, and fire is above air. Based on this arrangement, it appears Galileo assumes a Ptolemaic system that places Earth at the center of the universe, despite his later acknowledgment of Nicolaus Copernicus’s De revolutionibus orbium coelestium in Chapter 20.