“Astral” Beings may thus be defined in the same way as “material objects”; they are the Unknown Causes of various observed effects. They may be of any order of existence. We give a physical form and name to a bell but not to its tone, though in each case we know nothing but our own impressions. But we record musical sounds by a special convention. We may therefore call a certain set of qualities “Ratziel”, or describe an impression as “Saturnian” without pretending to know what anything is in itself. All we need is to know how to cast a bell that will please our ears, or how to evoke a “spirit” that will tell us things that are hidden from our intellectual faculties.
Aleister Crowley, Book 4, Notes for an Astral Atlas