'Second-order cybernetics' denotes a progressive intellectual movement within cybernetics and systems thinking that reached fruition in the 1970's. The most common explanation for the movement's agenda is that it turned the principles of cybernetics upon cybernetics itself. In second-order cybernetics, the observer of a system must be described and explained as well as the system observed. Key thinkers affiliated with second-order cybernetics include: von Foerster, Pask, Maturana, Varela, and Spencer Brown.