Such a miserable disappointment I scarce ever remember to have heard of. The small distance betwixt me and perfect health makes me the more uneasy in my present situation. It is a weakness rather than a lowness of spirits which troubles me, and there seems to be as great a difference betwixt my distemper and common vapours, as betwixt vapours and madness. I have noticed in the writings of the French mystics, and in those of our fanatics here, that when they give a history of the situation of their souls, they mention a coldness and desertion of the spirit, which frequently returns and some of them, at the beginning, have been tormented with it many years. As this kind of devotion depends entirely on the force of passion, and consequently of the animal spirits, I have often thought that their case and mine were pretty parallel, and that their rapturous admirations might discompose the fabric of the nerves and brain, as much as profound reflections, and that warmth or enthusiasm which is inseparable from them.
David Hume, A Letter to a Physician. It would be interesting to know which French writers in particular he had in mind. Jean-Jacques Olier's Journée chrétienne, which was very popular, is a possibility.