Illustrations of Institutio astronomica juxta hypotheses tam vetam quam Copernici and Tychonis

According to Gabriel Daniel, Gassendi was a little Pyrrhonian in matters of science; but that was no bad thing.[31] He wrote against the magical animism of Robert Fludd, and judicial astrology.[32][33] He became dissatisfied with the Peripatetic system, the orthodox approach to natural philosophy based on the writings of Aristotle. Gassendi shared the empirical tendencies of the age. He contributed to the objections against Aristotelian philosophy, but waited to publish his own thoughts.

There remains some controversy as to the extent to which Gassendi subscribed to the so-called libertinage érudit, the learned free-thinking that characterised the Tétrade, the Parisian circle to which he belonged, along with Gabriel Naudé and two others (Élie Diodati and François de La Mothe Le Vayer). Gassendi, at least, belonged to the fideist wing of the sceptics, arguing that the absence of certain knowledge implied the room for faith.