Daniel Kahneman opens Thinking, Fast and Slow with his foundational, enormously influential framework: the mind operates through two distinct systems — System 1, fast, automatic, and intuitive, and System 2, slower, more effortful, and deliberately analytical — with most everyday judgment and decision-making relying predominantly on the faster, more error-prone System 1.

Kahneman's foundational framework:

• System 1 operates automatically and rapidly, requiring minimal conscious effort, handling most routine perception, immediate reaction, and intuitive judgment without requiring deliberate, conscious reasoning
• System 2 operates more slowly and effortfully, requiring genuine conscious attention and deliberate reasoning, engaged for more complex analytical tasks but also, crucially, for monitoring and potentially correcting System 1's rapid intuitive judgments
• Kahneman argues that System 2, despite its capacity for more careful, deliberate reasoning, is frequently "lazy" — tending to accept System 1's rapid intuitive judgments without engaging its more effortful analytical capacity to verify these judgments, even when they're objectively incorrect

This foundational two-systems framework, developed through Kahneman's decades of research alongside his collaborator Amos Tversky, sets up the book's overall project: examining specific cognitive biases and errors that result from this reliance on System 1's fast, intuitive but frequently biased judgment, across numerous specific decision-making domains.