In
mathematics, the rejection of the proof of propositions and objects by reductio ad absurdum (i.e., by the demonstration that its negation would lead to contradictions) and instead the acceptance of the condition that objects be constructable from known elements by a
finite number of explicit procedures, e.g., by an
algorithm. One consequence of this is the denial of the universal validity of the
law of the excluded middle and the position of a third truth-value for classes of objects that are not so constructable: undecidable. 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 and acceptable for
simulation by a
computer .
cybernetics prominently participates in this movement by insisting that (1) all knowledge is created, invented and constructed by an observer, using his finite resources, that (2) the
cognitive system is organized or organizes itself so as to compute a stable reality (von Foerster) which implies that the constructs that do survive the circular process involving an observer and his environment are those that remain unaffected by disturbances in the form of
data which may enter the cycle involuntarily, and that (3) constructs, being descriptions (not necessarily of something else), may be communicated among observers who may include each other in their observations, (
see principle of relativity ) thus constituting an autonomous
organization involving a community of observers (Maturana) (
see communication, constitution ,
autopoiesis ,
social system ). (
Krippendorff )
