Leibniz's dispute with Newton over the physico-mathematical theories expounded in the Principia Mathematica (1687) have long been identified as a crucial episode in the history of science. Bertolini Meli here examines several hitherto unpublished manuscripts in Leibniz's hand illustrating his first reading of and reaction to Newton's Principia. Six of the most important manuscripts are here presented for the first time. Contrary to Leibniz's own claims, this new evidence shows that he had studied Newton's masterpiece before publishing An Essay on the Causes of Celestial Motions. This article,...