The problem of Galileo’s logical methodology has long interested scholars. In this volume, the author offers a solution that is backed by documentary evidence. His analysis starts with an early notebook Galileo wrote at Pisa, appropriating a Jesuit professor’s exposition of the Posterior Analytics of Aristotle, and ends with one of the last letters Galileo wrote, stating that in logic he had been a Peripatetic all his life. The author unearths the logic course from which the notebook was excerpted, then proceeds to show how its terminology and methodology continue to surface in Galileo’s later writings in which he founds his new sciences of the heavens and of local motion. The result should be of interest not only to Galileo’s scholars and logicians, philosophers and historians, but to anyone interested in the epistemic roots of modern science.