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I'm afraid I can't let you do that Dave.
— HAL ‧ 2001: A Space Odyssey
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
With courteous regret, the speaker refuses to permit what the listener, Dave, wants to do. The polite opening 'I'm afraid' cushions a flat denial, so the calm manner makes the refusal feel more final and unyielding than an angry no would.
Context reviewed