Spring 2016, Session 1: Galileo Galilei

Wednesday, January 27: We began the second semester of Philosophy of Nature: A History discussing the way that Galileo Galilei (1564-1642) contributes to the progression of Natural Philosophy — or rather, as Dr. Kane put it, how he gives with one hand and takes away with the other.  Aided by the telescope he developed in 1609, Galileo gave us a greater understanding of how universe functioned, but at the same time, he broke the relationship between nature and human nature in the following way: he asserted that sense experience is valid, but that it can deceive. In other words, there exists a true essence to the universe, but that true essence is invisible and never fully knowable in this present life.

In his Letters on Sunspots (1613) Galileo notes certain qualities of sunspots that he’s observed: they are variable in shape, the move about, they are not permanent, etc. He also notes that these sunspots turn around the sun, and theorizes that either they are carried along by the rotation of the sun itself, or that they move about a still solar body, carried by the its atmosphere.  Although these findings, that celestial bodies have imperfect and changeable characteristics, seem to contradict Aristotle’s conception of the heavens, Galileo maintains that the older philosopher would actually accept alternate conceptions now that telescopes have made it possible to prove them through observation.  Aristotle, after all, believed that Nature was discernible through observation.

On the topic of observation, in 1623 Galileo published Il Saggitore (“the assayer” or “the tester”) in which he delineates two types of perceptible properties: primary and secondary.  Roughly put, primary properties are objective or mathematically measurable (size, shape, motion) and secondary properties are subjective and depend on how each person perceives them (tastes, sounds, odors, etc.)  The idea that not everything is objectively perceptible is the beginning of the epistemological problem: how can we know?

To the extent that we can know, Gallileo depends on mathematics: “[The Book of Nature] is written in a mathematical language, and its characters are triangles, circles, and other geometrical figures; without these it is humanly impossible to understand a word of it, and one wanders around pointlessly in a dark labyrinth.”

For the next session we are reading Descartes’s Discourse on the Method and excerpts from Bacon’s Great Instauration and Novum Organum.  We’ll see how these philosophers respond to Galileo’s questioning of realism and perception.


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