From c. 1300 in the general sense "legal validity," also "authoritative book; authoritative doctrine" (opposed to reason or experience); "author whose statements are regarded as correct." From mid-14c. as "right to rule or command, power to enforce obedience, power or right to command or act." In Middle English also "power derived from good reputation; power to convince people, capacity for inspiring trust." From c. 1400 as "official sanction, authorization." Meaning "persons in authority" is from 1610s; Authorities "those in charge, those with police powers" is recorded from mid-19c.
mid-14c., auctor, autour, autor "father, creator, one who brings about, one who makes or creates" someone or something, from Old French auctor, acteor "author, originator, creator, instigator" (12c., Modern French auteur) and directly from Latin auctor "promoter, producer, father, progenitor; builder, founder; trustworthy writer, authority; historian; performer, doer; responsible person, teacher," literally "one who causes to grow," agent noun from auctus, past participle of augere "to increase," from PIE root *aug- (1) "to increase."
From late 14c. as "a writer, one who sets forth written statements, original composer of a writing" (as distinguished from a compiler, translator, copyist, etc.). Also from late 14c. as "source of authoritative information or opinion," now archaic but the sense behind authority, etc. In Middle English the word was sometimes confused with actor. The -t- changed to -th- 16c., on model of change in Medieval Latin, on mistaken assumption of Greek origin and confusion with authentic.
...[W]riting means revealing oneself to excess .... This is why one can never be alone enough when one writes, why even night is not night enough. ... I have often thought that the best mode of life for me would be to sit in the innermost room of a spacious locked cellar with my writing things and a lamp. Food would be brought and always put down far away from my room, outside the cellar's outermost door. The walk to my food, in my dressing gown, through the vaulted cellars, would be my only exercise. I would then return to my table, eat slowly and with deliberation, then start writing again at once. And how I would write! From what depths I would drag it up! [Franz Kafka, "Letters to Felice," 1913]