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I'm pretty sure that having to account for segfaults, keyboard interrupts, power outages, and other kinds of 'real world' exceptions that have to be caught means that the code might be deterministic but the computer is exposed to unexpected stimuli.

People probably act in a mostly deterministic when there are rules constraining them: sports, board games, social environments, lines. Maybe we are deterministic on a neurotransmitter level. But there's little control over external stimuli.