Aristotle's Posterior Analytics and Galileo Galilei: “Necessary Demonstrations”, “Demonstrative Regress and the Moon like Another Earth

Abstract

The author analyses the impact of Aristotle’s Posterior Analytics on Galileo’s argumentation in the Starry Messenger in favor of his astronomical and cosmological conclusions, based on the telescopic observations. After the summary of the third disputation of Galileo’s Tractatio de demonstratione on the questions arising from the Posterior Analytics, the author focuses on two issues: (1) What exactly is the meaning of Galileo’s “necessary demonstrations” or “demonstrative and necessary reasons”? (2) Can the “demonstrative regress” illuminate his argumentation? According to the author the example of the existence of mountains and valleys on the Moon shows that the influence of Florentine’s youthful notes is not detectable in his use of the method of “demonstrative regress” but in his conception of “necessary demonstration”.  Demonstration connects subjects (things) and predicates (properties). Natural, i.e. necessary predicate (property) of the subject can be grasped on the basis of the induction and experience, with the help of the “light of the intellect”. The intellect establishes the necessary connection of the subject (thing) and its predicate (property). Galileo’s Tractatio does not, however, reveal anything about the essential element of his demonstrations: “the necessity of geometrical demonstration”.