Our mind decides using one mode that's fast and intuitive, and another that's slow and deliberate.
Recognizing an angry face is instant. Solving a two-digit multiplication isn't. That difference is the foundation of the dual-process model of thinking. We don't make all our decisions the same way.
System 1 is fast, intuitive, and automatic. It operates with little or no conscious effort, and it usually gets us to "good enough" solutions without evaluating every alternative. System 2 is slow, deliberate, and analytical. It requires conscious effort, and it's what we use when we're genuinely searching for the best possible option, not just an acceptable one.
Most of our daily decisions are handled by System 1, including many we believe we thought through carefully. System 2 kicks in when a problem is new, complex, or when we consciously decide to switch off autopilot.
Using System 2 for every decision would be exhausting and unsustainable. Delegating most decisions to System 1 frees up cognitive effort, though at the cost of being more vulnerable to mental shortcuts and systematic errors.
Source: Stanovich, K. E. & West, R. F. (2000). Individual Differences in Reasoning: Implications for the Rationality Debate. Behavioral and Brain Sciences, 23(5), 645-665. Popularized by Kahneman, D. (2011). Thinking, Fast and Slow.
Most purchase decisions, app usage, or responses to institutional communication happen in System 1 mode. Designing with that in mind (reducing friction, making what's relevant visible) tends to work better than appealing only to rational arguments aimed at System 2.
This concept underlies our article What Is Behavioral Science?
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