Aleister Crowley, 1875-1947, British occultist, mystic, poet, mountain climber, sexual revolutionary, and social critic. He was involved in the Hermetic Order of the Golden Dawn, where he made enemies of William Butler Yeats and Arthur Edward Waite; later he came to head the Ordo Templi Orientis. He founded the religion (?) Thelema, based upon the text Liber Al vel Legis, or the Book of the Law, which allegedly was revealed to him by the spirit Aiwaz.
Crowley claimed to use the scientific method to study what people at the time called 'spiritual' experiences. Unfortunately, his work seems to have undergone little peer review; few if any have criticized it from a scientific standpoint.
Crowley was notorious in his life, and was a frequent target of attacks in the popular press. He was expelled from Italy under Mussolini after establishing at Cefalu a sort of magical commune.
Crowley's magical and initiatory system has in its innermost reaches a set of teachings on sex magick. He frequently expressed views about sex that were radical for his time, though not especially shocking by 21st century standards. He did not reveal his bisexuality except under pseudonyms.
Crowley also tried to mint a number of new terms instead of the established ones he felt inadequate. For example he spelled magic "magick" and renamed theurgy "high magic" and thaumaturgy "low magic". Many of his terms are still used by some practitioners.
Crowley remains an popular icon of libertines and those interested in the theory and practice of magic.