"What that power is by which the conscious spirit governs and directs various mental faculties, is, it must be confessed, utterly inexplicable as long as our souls are enclosed in material frames. While a watch is shut up in its case, we cannot see how the operations of its curious machinery are carried on; and the operations of the mind may, I think, be very well assimilated to those of a watch, as that comparison probably suggests the justest conception of what we can only fancy."
— Boswell, James (1740-1795)
See also James Boswell, The Hypochondriack, ed. Margery Bailey, 2 vols. (Stanford UP, 1928)