Pierre Gassendi (1592-1655) was a French philosopher, priest, scientist, astronomer, and mathematician. He spent most of his time in Paris, where he was a leader of a group of free-thinking intellectuals. He was an active observational scientist, publishing the first data on the transit of Mercury in 1631. The lunar crater Gassendi is named after him. He clashed with his contemporary Descartes on the possibility of certain knowledge. He revived Epicureanism as a substitute for Aristotelianism, attempting in the process to reconcile mechanistic atomism with the Christian belief in an infinite God.