Aristotle’s Physics and his treatise On the Heavens profoundly shaped scientific thought for over two millennia, dominating Western understanding of the natural world until the time of Galileo. While modern science has largely superseded his specific theories, understanding his imaginative background and conceptual framework is crucial for grasping the historical development of scientific inquiry. Unlike modern physicists who view the body of an animal as an elaborate machine, Aristotle and many Greeks found it more natural to assimilate seemingly lifeless motions to those of animals, believing that objects had an internal principle of motion, a “nature” ($\text{phusis}$) that drove their development towards a specific end.
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For Aristotle, physics was the science of “nature,” a word with a teleological implication. He believed that things existing by nature (animals, plants, elements) possessed an internal principle of motion or rest, guiding them towards their fulfillment. This differed from the purely mechanical views of the Atomists, as Aristotle argued that natural processes happened “for the sake of something.” He rejected the void, famously maintaining that “nature abhors a vacuum,” and engaged in a curious discussion of time, suggesting it was motion that admitted of numeration. His physics culminated in the argument for an unmoved mover, an eternal, unmovable substance at the circumference of the world that directly caused circular motion, the primary and most perfect kind of movement.
🌕 What Was Aristotle’s Cosmic Order?
Aristotle’s cosmology, detailed in On the Heavens, presented a beautifully simple yet ultimately flawed theory of the universe. He posited a fundamental division: the sublunary sphere (below the moon), where everything was subject to generation and decay and composed of the four terrestrial elements (earth, water, air, fire); and the celestial sphere (from the moon upwards), where everything was ungenerated, indestructible, and composed of a fifth element, the incorruptible aether. The Earth, spherical and at the center of the universe, was surrounded by perfectly spherical heavens, with stars and planets fixed on rotating spheres. The natural movement of terrestrial elements was rectilinear (up or down), while that of the fifth element was circular, reflecting its divine perfection. This geocentric and geostatic model, while aesthetically pleasing and seemingly intuitive, provided many difficulties for later ages, particularly with the observation of comets and the development of heliocentric theories.
💥 How Did Galileo and Newton Challenge Aristotle’s Physics?
Aristotle’s physics, with its emphasis on purpose and its distinction between terrestrial and celestial realms, proved to be a significant obstacle to scientific progress. His belief that a projectile fired horizontally would move horizontally for a time and then suddenly fall vertically was famously refuted by Galileo’s discovery of parabolic trajectories. More fundamentally, Aristotle’s physics was incompatible with Newton’s First Law of Motion (the law of inertia), which states that a body in motion will continue in a straight line with uniform velocity unless acted upon by an external force. This revolutionary idea removed the need for continuous divine intervention to explain motion and established a purely mechanical view of the universe. The abandonment of Aristotle’s eternal and incorruptible heavenly bodies, and the realization that all visible matter is subject to change and decay, further cemented the triumph of modern science over his long-dominant physical theories. Despite its eventual refutation, Aristotle’s physics provided a comprehensive framework that stimulated inquiry and served as a crucial intellectual target for the scientific revolution.
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Russell, Bertrand. A History of Western Philosophy. Simon and Schuster, 1945.
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