authentikos

Greek authentes combined autos (self) and hentes (an agent, one who acts). An authentes was one who acted on their own authority — an original agent, the person responsible for an action. Authentikos described what derived from such an original authority: genuine, not copied, not fake. An authentic document was one whose authority came from the original source rather than through intermediaries.

Medieval Latin used authenticus for official documents and genuine texts — those whose authority came from the original rather than from copies. Authentic was the opposite of forgery or derivative copy. The church needed to distinguish authentic scripture from apocryphal texts, authentic relics from fraudulent ones. Authentication was the process of verifying original authority.

The existentialist tradition transformed authenticity from a documentary property into a personal one. Kierkegaard, Heidegger, and Sartre all used authenticity for the condition of living genuinely from one's own authority rather than from social conformity or bad faith. Heidegger's authentic Dasein was the person who owned their own existence, accepted mortality, and acted from their own understanding rather than from the anonymous 'they' of social pressure.

Today authenticity is a marketing concept as much as a philosophical one. Brands claim to be authentic; social media influencers cultivate authentic personas; the authentic dining experience commands a premium. The Greek original-authority has become a cultural value contested between genuine self-expression and performed naturalness — which is to say, precisely what the existentialists worried about.