Abstract

Published exactly four centuries ago, Galileo’s Il Saggiatore (1623) originated from a dispute with Jesuit mathematician Orazio Grassi over the nature and motion of comets that appeared in the second half of 1618. Il Saggiatore, however, is much more than a work on comets. In numerous long digressions, Galileo analyzes various physical phenomena, where he expounds, for example, his conception of the structure of matter, based on the distinction between primary and secondary qualities, and explains the corpuscular nature of heat. In this way, Galileo took the opportunity to present his new natural philosophy, which, through a rigorously mathematical (according to the famous image of the book of nature) and empirically grounded approach, was a viable alternative to the traditional Aristotelian-scholastic system. The paper also shows how the comet debate had obvious cosmological implications, with Galileo’s attempt to give new life to Copernicus’ heliocentric theory, and emphasizes that Galileo ascribed to his comet theory not a value of absolute truth, but a hypothesis that could explain observational data