"The sound of the mind we hear; but what it is we cannot tell. The music which it utters, its melody, its harmony, its discord, its variety of notes, have been written by Shakespeare with a wonderful degree of perfection, so as to be themselves to every cultivated reader. We have even gamuts and treatises of the grounds of its music--witness a Locke and other metaphysicians. But the instrument is as much concealed from our intelligence, as the spheres of which the delightful music has been fancied by romantic imaginations."
— Boswell, James (1740-1795)
(I, pp. 151-3 in SUP edition)
See also James Boswell, The Hypochondriack, ed. Margery Bailey, 2 vols. (Stanford UP, 1928)