“Indubitably, Magick is one of the subtlest and most difficult of the sciences and arts. There is more opportunity for errors of comprehension, judgement and practice than in any other branch of physics.”
Source: The Confessions of Aleister Crowley: An Autohagiography
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Aleister Crowley 142
poet, mountaineer, occultist 1875–1947Related quotes

“Magick is the Science and Art of causing Change to occur in conformity with Will.”
Introduction.
Source: Magick Book IV : Liber ABA, Part III : Magick in Theory and Practice (1929)
Context: Magick is the Science and Art of causing Change to occur in conformity with Will.
(Illustration: It is my Will to inform the World of certain facts within my knowledge. I therefore take "magical weapons", pen, ink, and paper; I write "incantations" — these sentences — in the "magical language" ie, that which is understood by the people I wish to instruct; I call forth "spirits", such as printers, publishers, booksellers and so forth and constrain them to convey my message to those people. The composition and distribution of this book is thus an act of Magick by which I cause Changes to take place in conformity with my Will.)
In one sense Magick may be defined as the name given to Science by the vulgar.

“The practice of physic is jostled by quacks on the one side, and by science on the other.”
Book I, p. xxv
Collected Works

Source: De architectura (The Ten Books On Architecture) (~ 15BC), Book I, Chapter I "The Education of the Architect" Sec. 1

Source: "The Latest Attack on Metaphysics" (1937), p. 133.
“Any sufficiently advanced form of magick will appear indistinguishable from science.”
Source: PsyberMagick (1995), p. 15

1947
Earth, Inc. (1973) ISBN 0-385-01825-8 This is just part of a very long sentence that covers the whole first page, but in this part of the quote, the intention of the entire book is stated.
1970s

Source: The systems view of the world (1996), p. 8 as cited in: Martha C. Beck (2013) "Contemporary Systems Sciences, Implications for the Nature and Value of Religion, the Five Principles of Pancasila, and the Five Pillars of Islam," Dialogue and Universalism-E Volume 4, Number 1/2013. p. 3 ( online http://www.emporia.edu/~cbrown/dnue/documents/vol04.no01.2013/Vol04.01.Beck.pdf).