William James, The Principles of Psychology, Volume 1…
“ On the one hand, they advise you to isolate yourself, as far as possible, from every external sensation, especially every intellectual work,—for if you were to busy yourself even with the simplest calculation, what would become of internal observation?—on the other hand, after having with the utmost care attained this state of intellectual slumber, you must begin to contemplate the operations going on in your mind, when nothing there takes place! ”
