Galileo Galilei (1564–1642), Italian astronomer, natural philosopher, and physicist. His Dialogue concerning the Two Chief World Systems (1632) defended Copernicus by arguing against the major tenets of the Aristotelian cosmology. On his view, one kind of motion replaces the multiple distinct celestial and terrestrial motions of Aristotle; mathematics is applicable to the real world; and explanation of natural events appeals to efficient causes alone, not to hypothesized natural ends. Galileo was called before the Inquisition, was made to recant his Copernican views, and spent the last years of his life under house arrest. Discourse concerning Two New Sciences (1638) created the modern science of mechanics: it proved the laws of free fall, thus making it possible to study accelerated motions; asserted the principle of the independence of forces; and proposed a theory of parabolic ballistics. His work was developed by Huygens and Newton.
Galileo’s scientific and technological achievements were prodigious. He invented an air thermoscope, a device for raising water, and a computer for calculating quantities in geometry and ballistics. His discoveries in pure science included the isochronism of the pendulum and the hydrostatic balance. His telescopic observations led to the discovery of four of Jupiter’s satellites (the Medicean Stars), the moon’s mountains, sunspots, the moon’s libration, and the nature of the Milky Way. In methodology Galileo accepted the ancient Greek ideal of demonstrative science, and employed the method of retroductive inference, whereby the phenomena under investigation are attributed to remote causes. Much of his work utilizes the hypothetico-deductive method. R.E.B.