Ineffability. — The handiest of the marks by which I classify a state of mind as mystical is negative. The subject of it immediately says that it defies expression, that no adequate report of its contents can be given in words.
James establishes ineffability as the primary and defining criterion of mystical experience, grounding it in the impossibility of verbal report and the necessity of direct acquaintance.
, The Varieties of Religious Experience Amazon, 1902thesis