 It is likely that Hume did get to know the relevant editions of Opticks already during his years spent at the University of Edinburgh, and may well have been exposed to lectures on it as well, but it is uncertain how reliable his working knowledge of it was (Barfoot 1990, esp. 158 and 152). Although in general he was dissatisfied with university education, at least he found some satisfaction in his natural philosophy class taught by Robert Steuart (Stewart 2005: 21–25). A probably more important introduction to the outlook that has spread due to the influence of the Opticks came from Hume’s medical readings in relation to his mental breakdown at the age of 18. The most important document of this struggle is his 1734 “Letter to a Physician” (Letters 1:14–18.), in which the medical description of his condition, as M.A. Stewart (2005: 24n48) puts it, “seems to be consciously modelled on George Cheyne’s The English Malady”, published in 1733. A similarly likely source is Mandeville’s Treatise of the Hypochondriack and Hysterick Diseases (1st ed. 1711, 2nd ed. 1730, see Wright 2009: 8–9). In his latter life he probably benefitted from discussions with philosophical friends and colleagues, most notably with William Cullen and Joseph Black, these discussions, however, came only after the publication of Hume’s chief philosophical works. These friendships were indeed philosophical in the sense that they were founded, at least partly, on sympathies for Hume’s philosophical project (see, e.g., Hume’s letter to Cullen in Letters 1:163)