On the Langage of Theater

To practice is both to learn to do something, and to do something well (as in to practice medicine)

To "act" is to both do something, and to pretend to do something. If we act a certain way, after a while those actions can become actual. As Hamlet said to his mother, we should

"Assume a virtue if you have it not.
That monster custom, who all sense doth eat,
Of habits devil, is angel yet in this,
That to the use of actions fair and good
He likewise gives a frock or livery
That aptly is put on. Refrain [to-]night,
And that shall lend a kind of easiness
To the next abstinence, the next more easy;
For use almost can change the stamp of nature,
And either [curse?] the devil or throw him out
With wondrous potency." (Act 3, Scene 4)

To perform is both to do something, and to pretend to do something

The world is a stage on which we practice, perform, and act

"Theater" and "theory" related to each other through their Greek sources. The Greek ancestor of theater is theatron, "a place for seeing, especially for dramatic representation, theater." Theatron is derived from the verb theasthai, "to gaze at, contemplate, view as spectators, especially in the theater," from thea, "a viewing." The Greek ancestor of theory is theoria, which meant among other things "the sending of theoroi (state ambassadors sent to consult oracles or attend games)," "the act of being a spectator at the theater or games," "viewing," "contemplation by the mind," and "theory or speculation." The source of theoria is theoros, "an envoy sent to consult an oracle, spectator," a compound of thea, "viewing," and -oros, "seeing."

Merely a theory? How can you understand what you are seeing without one?