irrationally

Irrationally is an adverb that means performing an action in an irrational manner. It describes behavior, beliefs, or decisions that lack reason, evidence, or logical justification. The term is typically applied to human actions and thoughts rather than mathematical objects.

Etymology and grammar. The word is formed by adding the -ly suffix to the adjective irrational, which itself comes from Latin irrationalis (in- meaning not + rationalis meaning of reason). In usage, irrationally modifies verbs and clauses, as in acting irrationally or believing something irrationally. It is not used to describe numbers; for mathematical irrationality the adjective irrational is used, without the -ly form.

Usage and nuance. Irrationally often carries a evaluative nuance, signaling disapproval or surprise at a departure from rational judgment. In everyday language it is common in phrases like act irrationally, behave irrationally, or irrationally optimistic. In formal or scientific writing, writers may substitute more precise expressions such as “without sufficient justification,” “not supported by evidence,” or “in a non-logical manner” to avoid ambiguity.

Examples. She acted irrationally when she refused to consider any counterarguments. The plan failed because investors viewed it as irrationally risky. His irrationally firm belief in the hypothesis persisted despite contradictory data.

See also. Rationality, reason, logic, cognitive biases, and related discussions of decision-making and behavior.