irrationele

Irrationele is a Dutch-language adjective meaning not based on reason or logic, and it is used across contexts to describe beliefs, actions, or processes that fall outside standard norms of rationality. In mathematics the term is closely associated with irrational numbers, which cannot be expressed as a ratio of two integers and have non-terminating, non-repeating decimal expansions. The concept of irrationality distinguishes real numbers such as √2, π and e from rational numbers, which can be written as fractions. There are infinitely many irrationals, and they are dense among the real numbers, meaning between any two real numbers there is an irrational number.

Etymology: irrationele derives from Late Latin irrationalis, via Old French irrationnel, with the Dutch form adopting a final -e in agreement with noun phrases, and the root related to ratio, “reason.”

Philosophically and in everyday language, irrational describes judgments or behaviors that lack justification by reasons or evidence. In psychology and behavioral sciences, irrationality is studied as departures from normative models of rationality, often due to cognitive biases, emotions, or informational limits. In public discourse, calling a claim irrational implies it is not supported by logic or evidence, while a careful distinction is kept between irrational beliefs and clinical insanity.

See also: Rational number, Irrational number, Irrationality, Cognitive biases.