properly means one who has attained (from the Latin, adeptus, participle of adipiscor). The alchemists applied the term vere adeptus, to those persons who professed to have “attained to the knowledge of” the elixir of life or of the philosopher's stone.
Alchemists tell us there are always 11 adepts, neither more nor less. Like the sacred chickens of Compostella, of which there are only 2 and always 2—a cock and a hen.
In Rosicrucian lore as learn'd As he that vere adeptus earn'd.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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