Discoveries and Opinions of Galileo

Writing to his friend Don Virgilio Cesarini, Galileo complains of the “ill feeling” and “stubborn opposition” that have come against him and his scientific writings. Critics don’t seem to realize that Galileo’s findings come from the plain evidence of the senses, which they can see for themselves. Some jealous scientists, such as Simon Mayr, even tried to appropriate Galileo’s discoveries as their own. Most recently, a person calling himself “Lothario Sarsi” attacked Galileo in print in response to Galileo and Mario Guiducci’s work “Discourse on Comets.” While Galileo at first resolved to ignore these attacks and maintain a peaceful frame of mind, their persistence compelled him to speak out.

These are the main points that Galileo makes in reference to Sarsi:

- Sarsi relies on authority in philosophy and science. Instead, Galileo insists that experience and reason are the proper guides to the truth.

- Sarsi disparages Galileo’s theories because they have historically had few followers. Galileo responds that the number of people who understand true science and philosophy is small, while “the crowd of fools who now nothing…is infinite” (239).

- Sarsi criticizes Guiducci’s statement that the telescope magnifies objects to an infinite degree. Galileo shows that this is mere sophistry and quibbling over words, because “infinite” is used here in a relative sense (241-42).

- Sarsi’s criticism of Guiducci’s description of how the telescope magnifies the image of stars is likewise an idle quibbling over words (243).

- Sarsi tries to rob Galileo of the honor of having been the first to adopt the telescope, calling it merely Galileo’s “foster child.” Galileo reviews and defends his role in the discovery of the instrument (243-44).

- Sarsi claims that objects seen through a telescope appear more enlarged the closer they are. Galileo demonstrates the falsity and absurdity of this, since it is by changing the length between the lenses that things appear nearer or more distant (247-52).

- Sarsi argues that comets are like planets. Galileo shows that Sarsi’s argument is weak and that comets are made from thin and tenuous material, like fog or smoke, which makes them very different from planets in composition (253-54).

- Sarsi criticizes Guiducci and Galileo for declaring that a comet’s motion is necessarily straight and perpendicular to the earth’s surface. Galileo points out that he and Guiducci did not definitely argue this, but merely posed it as a possibility to explain the observed changes in position of the comet (254).

- Sarsi shows undue confidence in the testimony of his senses concerning the nature of the comet’s light. Galileo points out that our senses are often deceptive (255).

- Galileo points out that Sarsi too often shapes his premises to fit a preconceived conclusion, instead of the other way around (254).

- Galileo explains how distant luminous bodies like the sun or a comet reflect their light to us from afar (258-61).

- Sarsi claims that Galileo espoused a theory about comets’ tails that was previously refuted by Johann Kepler. Galileo shows that the proposition he espoused was not the same as the proposition that was refuted by Kepler (261-62).

- Galileo mocks the idea, derived from Aristotle, that the sky must have a uniform shape because it is the “noblest body” (263). Likewise, that the moon and planets are rough in surface is proven by the fact that we see their reflections; a smooth surface would not reflect light to us so well.

- Galileo explains how he refuted Copernicus’s theory of a “third motion” of Earth by the principle of inertia and contrary motion (264-65).

- Galileo explains that friction, not motion, is the cause of heat. Heat, in fact, is merely a sensation and not something real (266-77).