In ‘THE END OF FAITH’ Sam Harris writes:
“The history of human spirituality is the history of our attempts to explore and modify the deliverances of consciousness through methods like fasting, chanting, sensory deprivation, prayer, meditation, and the use of psychotropic plants. There is no question that experiments of this sort can be conducted in a rational manner. Indeed, they are some of our only means of determining to what extent the human condition can be deliberately transformed. Such an enterprise becomes irrational only when people begin making claims about the world that cannot be supported by empirical evidence.” (chapter 7, ‘Experiments in Consciousness’, page 210)