In mathematics, rejection of proof of propositions by reductio ad absurdum (ie by demonstration that its negation would lead to contradictions) ... one consequence is the denial of the universal validity of the law of the excluded middle.
In cognitive psychology, constructivism is a reaction against the Gibsonian view that knowledge and perception are the result of sensation, and maintains that: (a) the nervous system, in order to be adaptive, must process available information actively and construct an internal world and that (b) these processes be describable in a computer language. Cybernetics participates in this movement by insisting that (1) all knowledge is created by an observer using his finite sources and (b) that the cognitive system is organized or organizes itself so as to compute a stable reality which implies that the constructs that do survive the circular process involving the observer and his environment are those that remain unaffected by disturbances in the form of data which may enter the cycle involuntarily; and that (c) constructs, being descriptions, may be communicated between observers who may include themselves in these observations thus constituting an autonomous organization.