 Third, Hume would have been reasonably familiar with Newton’s Opticks, especially his writings on colors. It is less clear how thorough his working knowledge of it was (see Schliesser 2004 for some concerns). However, it is less likely that he was exposed to most of the technically challenging parts of Principia. As the catalogue at La Flèche testifies, there was four entries directly related to Newton, of which only one, a 1720 French edition of Newton’s Opticks (but no edition of Principia), refers to an edition that could have been there during Hume’s stay (Perinetti 2018: 51). Moreover, if Hume never read the third edition of the Principia, then this could help explain his lack of interest in Newton’s fourth rule of reasoning which was added to the third edition (see  section 4.5,  Rules of Reasoning).