These divergent definitions suggest an interesting tension within our understanding of what it is to perform, perhaps especially in a world where we presume everything is being done for show. A performance is, by definition, something false, put on, artificial, self-conscious. And also, by the opposing definition, something authentic, persuasive, organic, true.

The illusion they create isn’t that they really are who they are playing, but rather that, whoever they are, we know them.

In his book “The Method,” which will be published early next year, the critic and stage director Isaac Butler traces the history of this tension as it applies to acting. Starting in prerevolutionary Russia, a new approach to theater insisted on truth — as opposed to eloquence, bravura or technical skill — as the highest value in acting. Its guru was Konstantin Stanislavsky. The Russian word perezhivanie, usually rendered as “experience” and described by Butler as “a state of fusion between actor and character,” was the key to Stanislavsky’s system.

The experience of the character is what the actor explores inwardly and communicates outwardly, in such a way that the spectator accepts what he or she knows is not the case. We don’t mistake Will Smith for Richard Williams, Kristen Stewart for Diana or Bo Burham for himself, but we nonetheless believe them.

The arrival of Stanislavsky’s teaching in America — where it was preached as the Method by teachers like Harold Clurman, Lee Strasberg and Stella Adler and practiced by artists like Elia Kazan, Marlon Brando and Kim Stanley — coincided with a renewed commitment to realism in theater and film. For actors, the always elusive, you-know-it-when-you-see-it standard of realism was not faithful mimicry so much as psychological truth. There were differing ideas about how that could be achieved, but a basic tenet was that the feelings, memories and impulses of the performer were tools for mastering the character.

The Method peaked in the 1950s and ’60s, but the mystique of authenticity remains. In popular culture, “method acting” now refers to an extreme commitment to erasing the boundary between character and self, a kind of total identification that is in many respects the opposite of what Stanislavsky and his American followers espoused. It means throwing yourself headlong into a character: speaking in dialect 24/7; gaining or losing a lot of weight; embracing outlandish behavior; neglecting personal hygiene. Not to find the sources of the character within yourself, but to make yourself, almost literally, into the character, to go so far into the performance that you are no longer performing.

If you follow that logic far enough, it starts to loop back on itself. Didn’t we already establish that everyone is always performing? Doesn’t that make every performance a meta-performance? Isn’t authenticity another kind of artifice?