SPOCK: Incredible power. It can’t be a machine as we understand mechanics.
KIRK: Then what is it?
GUARDIAN: (The doughnut pulses bright in time with the words) A question. Since before your sun burned hot in space and before your race was born, I have awaited a question.
KIRK: What are you?
GUARDIAN: I am the Guardian of Forever.
KIRK: Are you machine or being?
GUARDIAN: I am both and neither. I am my own beginning, my own ending.
SPOCK: I see no reason for answers to be couched in riddles.
GUARDIAN: I answer as simply as your level of understanding makes possible.

The City on the Edge of Forever