Early-Modern Irish Catholics exiled on the European continent are known to have often held prominent academic positions in various important colleges and universities. This paper investigates the hitherto unknown Scholastic legacy of the Dublin-born Jesuit John Austin (1717–84), a famous Irish educator who started his career teaching philosophy at the Jesuit college of Rheims in 1746–47, before returning to the country of his birth as part of the Irish Mission. These manuscript lecture notes provides us first-hand knowledge about the content of French Jesuit education in the middle of the eighteenth century, which does not correspond to its classical reputation of ‘Aristotelian’ scholasticism opposed to philosophical novelties. While stitching to a traditional way of teaching, Austin introduces positively elements from Descartes, Malebranche, Locke and Newton into the curriculum. The present paper focuses on his conception of philosophical certitude (certitudo), which he considered a necessary condition for the possibility of philosophical knowledge.