William Ross Ashby (September 6th 1903, London - November 15th 1972) is an English psychiatrist-engineer come very early to the Cybernétique and its influence is such as Ludwig von Bertalanffy mentioned it for its contribution which influenced Norbert Wiener, Herbert Alexander Simon and the others.

De Ashby, the Loi of the necessary variety is most known. The " variété" is the enumeration of the quantity of behaviors and different states exhibés by a system.

So that a system " A" can control (at the same time " surveiller" in the French significance and " commander" in the English significance) a system " B" , it is necessary and it is enough that the variety to " A" that is to say higher or at least equalizes with that of " B". It follows that a " inversion of contrôle" occurs when the variety of the " commandé" increase and exceeds that of the " commandeur". It is the cybernetic version of the " dialectical of the Master and the esclave" at Hegel. By work with the service of the Master, the slave acquires competences more and more while the Master loses his initial competences by be made be useful and, then, the slave becomes the Master of his former Master who becomes the new slave of his former slave!

Ashby worked much on the increase in the intelligence and takes part in work on the Artificial intelligence, the complexity connected to the variety and the origins of the order expressed by the principle of von Foerster " Order from Noise" and the principle of Schrödinger " Order from Order". The first is a stochastic process of the chance and need from the point of view of a CYBERNETIC EXPLANATION where a " answer appropriée" with the constraints of the environment and context has better chances of survival, of development and reproduction among all the possible occurrences and the second and a principle of organization by extension. The principle " Order from Noise" is also a principle of Car-organization and Complexité at Edgar Morin and Henri Atlan for which the condition of the car-organization is the " redondance" initial high which is not other than the " variété" of Ashby.