He distinguishes between a “sensible rapture” (sinnliche Wollust) and an emotion because the former begins in a part of a body, that is, with an actual external perception, while the latter “arises in the brain itself,” but in either case the feeling of the pleasure “arrange[s] the fibers of the brain into an appropriate tone, employing them without fatiguing them,” and then “The brain communicates this harmonious tension to nerves of the other parts of the body and the body becomes comfortable” (On Sentiments, Twelfth Letter; Philosophical Writings, p. 53). In other words, although as a rationalist metaphysician Mendelssohn maintains the formal distinction between the mind and the body (the mind is simple and indivisible, while body is essentially divisible), as a psychologist and aesthetician he nevertheless sees them as in the most intimate interaction, with the perception of harmony by the body infusing the mind with a pleasant sense of harmony that then further stimulates the harmonious condition of the body.