Galileo was born near Pisa and died at Arcetri, near Florence. He taught mathematics at Pisa and then moved to the University of Padua in 1592. In 1610 he was appointed chief mathematic and philosopher to the Grand Duke (of Florence). In 1632 he published his Dialogue concerning the Two Chief World Systems. In 1633 he was convicted on suspicion of heresy. He abjured and was sentenced. In 1638 he published Discourses and Mathematical Demonstration concerning Two New Sciences.

Around 1590 Galileo reached a conclusion in De Motu about the inadequacy of Aristotle's conception of natural motion and the nature of matter (Galilei 1890–1909, vol. 1). In this unpublished work, he argued that all matter (at least in the sublunary world) was the same – there was no light matter that naturally moved up. All matter moved down by weight (or specific gravity). For the next twenty years he would attempt to develop these ideas into numerous insights about the nature of motion and unitary nature of matter.

In March 1610, Galileo published Sidereus Nuncius (The Starry Messenger) in which he reported his telescopic observations. He claimed that there were mountains on the moon, that there were numerous stars hitherto unseen, and that Jupiter had four moons. The observations served to support his denial of a celestial-terrestrial distinction. In April 1611, Cardinal Bellarmine wrote to the Roman College (Collegio Romano) asking it for an opinion on Galileo's observations. The members of the collegio responded quickly, affirming all of Galileo's observations, but noting that Father Christoph Clavius still had some doubts about the mountains on the moon. Galileo moved back to Florence (from Padua), accepting a position with the Medici as “Chief Mathematician of the University of Pisa and Philosopher to the Grand Duke.” In 1613 he published Letters on the Sunspots in which he first expressed his position in favor of Copernicus and discussed the phases of Venus and the rotation of spots on the sun.

Descartes does not often use the word “physics” (physica, physicos, or physique) in his writings. Perhaps his boldest and most famous pronouncement about physics comes in the preface to the 1647 French translation of his Principles of Philosophy (1644). There he explains what a man must do to apply himself to “True Philosophy.” He first must study metaphysics, and then, the

Parts II, III, and IV of the Principles are partly dedicated to “Physics,” yet well before 1644 Descartes was engaged with physical problems and seems to have begun his work in “physics” with his Dioptics, which was composed in the early 1630s. However, in his correspondence, the first work he sometimes refers to as “my Physics” is found in The World (see, e.g., AT I 348, CSMK 52). In both the physics of The World (1633) and of the later Principles, Descartes lays out a general account of matter and motion as well as a vortex theory of planetary motion. In what follows, the notable differences in how Descartes approaches some of the same issues in these two works are highlighted.

In The World, Descartes outlines his physics in a quite realistic manner. He lists three basic elements (fire, air, earth), which can be used to explain all natural phenomena by means of the “motion, size, shape and the arrangement of ... [their] parts” (AT XI 26, G 18). Descartes also writes:

Galileo's life and works, like Gaul, has generally been divided into three parts. There is his work on mechanics and local motion (with the science of the strength of bodies grudgingly admitted), his work on astronomy and Copernicanism, and, finally, his relations and tribulations with the Catholic Church. Occasionally, some scholars under the anachronistic rubric of methodology have attempted to tie the two scientific parts together to obtain a more coherent picture.

In the space of this essay, I cannot overcome this tripartite division that the centuries have sanctioned. I can, however, sketch a picture of Galileo that will be a step toward this goal. It seems to me that Galileo had only a few basic conceptions that directed his life and work in all these three areas. His first belief concerned the role of the properly thinking and working individual scientist as being able to obtain knowledge and certainty. This belief showed up in his writings about the role of the scientific elite individual, who saw and understood things that were not seen by the masses, or, especially, by groups dedicated to the authority of Aristotle. It also seems to lie at the heart of his thoughts about Biblical exegesis. This individualism is apparent in Galileo's texts by his ubiquitous and consistent use of the first person singular "I" ("io") and the use of proper names or descriptions to talk about insights, discoveries, and all the accomplishments of good science.