The finished product of a carpenter's work can be tried in use and tested by examination. His chairs can be sat upon, his table legs measured, and faults or inaccuracies will soon reveal themselves. But how are the mystic's intuitions, inspirations, visions, and teachings to be appraised, measured, tested with complete certainty? How much in them can be fully trusted, how much suspected as being the undivine part? The metaphysician's concepts and the religionist's beliefs come into the same category; they cannot at once be checked for faults, tried by results, or measured for accuracy, whereas the craftsman's productions can. Religion, mysticism, and metaphysics cannot immediately offer their proofs, if at all.
-- Notebooks Category 16: The Sensitives > Chapter 3: Philosophy, Mysticism, and The Occult > # 84