Rationaalisuus

Rationaalisuus, or rationality, is the quality of being reasonable or of acting in accordance with evidence, logic, and one's goals. In analytic use it splits into normative rationality—how agents ought to reason and decide—and descriptive rationality—how people actually reason in practice. A further distinction is epistemic rationality (beliefs supported by evidence and probability) and instrumental rationality (actions that align with one’s goals given one’s beliefs).

Normative theories include formal logic and probability theory, Bayesian rationality, and expected utility theory, which describe idealized ways to form beliefs and choose among options. In AI and statistics, Bayesian rationality describes beliefs updated by Bayes' rule. Bounded rationality, a concept introduced by Herbert A. Simon, recognizes cognitive limits and practical constraints, often leading to satisficing—seeking a satisfactory rather than optimal solution.

Descriptive approaches study actual human reasoning, highlighting cognitive biases and heuristics that depart from ideal models. Dual-process theories differentiate fast, intuitive thinking from slower, deliberative reasoning, with substantial research by Kahneman and Tversky showing predictable deviations from rational choice in risk, judgment, and decision making.

The concept has broad relevance in philosophy, economics, psychology, AI, and social sciences. It is central to rational choice theory, Bayesian inference, and decision making under uncertainty. Critics argue that overreliance on formal rationality can ignore social context, emotions, and values, while others defend the practical usefulness of rationality as a guiding ideal.