In Daniel Kahneman's "Thinking, Fast and Slow", System 2 refers to reasoned, conscious thought process, for example weighingnup options for a holiday or deciding what kind of ball to pitch in a game before actually throwing it. It is on contrast to System 1, which is the faster more unconscious reactions. System 2 is relatively slow, but open to explanation, communication and discussion. In AI, traditional symbolic AI in corresponds most closely to human System 2 thinking.
Defined on page 4
Used on pages 4, 538
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