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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| De motu antiquiora | 3/12 | https://en.wikipedia.org/wiki/De_motu_antiquiora | reference | science, encyclopedia | 2026-05-05T08:51:23.985272+00:00 | kb-cron |
=== Chapter 2: Heavy substances are by nature located in a lower place and light substances in a higher place === Galileo assumes a quasi-Aristotelean 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. Galileo indicates that Aristotle gave no reason for the arrangement of the classical elements other than that everything must be disposed in some arrangement as provided by divine providence; however, Galileo finds this view unsatisfactory and believes that Aristotle was potentially incorrect in his criticism of the ancient monist and atomist theories. He argues that, if the monists and atomists were correct, it would provide a logical explanation for the arrangement of the elements: heavier bodies would enclose more particles of that matter in a narrower space and would also occupy narrower places, such as those near the center of the universe (Galileo claims that spaces become narrower as we approach the center of a sphere without further elucidating his meaning). For example, earth elements occupy a small amount of space, whereas air elements occupy an ample amount of space. Galileo concludes that the arrangement of the universe isn’t randomly chosen, but is executed with both prudence and justice.
=== Chapter 3: Natural motion is caused by heaviness or lightness === Bodies (composed of the classical elements) are at rest when at their proper place, but when displaced above a lighter body, they will move down below the lighter body unless the lighter is forced to remain under the heavier. The converse applies to lighter bodies. For “natural” motion (as opposed to “violent” upward-projectile motion), both the heaviness/lightness of the body and the heaviness/lightness of the medium are to be compared, for if water were not lighter than stone, a stone would not sink in water. Galileo reemphasizes that heaviness/lightness (i.e., density) should be observed strictly as previously defined, as it’s not the weight of the entire body of water that is considered, but rather the weight of a portion of the water equal in volume to the body that is passing through the water. Motion is then correlated to the relative heaviness between two bodies, which Galileo sets out to prove in subsequent chapters.
=== Chapter 4: Proof that bodies of the same heaviness as the medium move neither upward nor downward === Said proof is provided.
=== Chapter 5: Proof that bodies lighter than water cannot be completely submerged === After providing said proof, Galileo concludes that it’s obvious that bodies heavier than water are necessarily submerged (for if it weren’t, then it would be lighter than water, and therefore contrary to its assumption) and must continue to move downward (for if they did not, then it must have equal or less weight than water). Moreover, since bodies that move downwards must be heavier than the medium, it can be said that heavy bodies move downward by reason of their weight.
=== Chapter 6: An analogy between bodies moving naturally and the weights of a balance === Since natural motion results due to the heaviness/lightness of the medium and the body, and since the respective heaviness/lightness can be compared through respective weights with equal volumes, Galileo recognizes that the same can be said of weights on a balance, and that, in viewing the lever as an analogy for motion, it can be easily understood why solids lighter than water (e.g., wood) are not completely submerged in water – the heavier cannot be raised by the heavy. Under this assumption, the cause of motion for bodies moving naturally (in the same manner as weights in balance), both up and down, can be referred to weight alone. Through this lever analogy, a mobile moves by force and by the extruding action of the medium. For when wood is forcibly submerged, the water thrusts the wood back out when the water moves towards its own proper place. In the same way, a stone in freefall is thrust from its position and is impelled downward because it is heavier than the medium. Thus, natural motion may be considered “forced.”