William Ross Ashby (September 6, 1903, London - November 15, 1972) was an English psychiatrist and a pioneer in the study of complex systems. Despite being widely influential within cybernetics, systems theory and, more recently, complex systems, he is not nearly as well known as many of the notable scientists his work has influenced including Herbert Simon, Norbert Wiener, Ludwig von Bertalanffy, Stafford Beer and Stuart Kauffman.
In May 1928, Ashby was 24 and a medical student at St Bartholomew's Hospital in London when he started recording his theories in a series of 25 notebooks. He wrote on average half a page a day for 44 years until shortly before he died. In January 2003 the notebooks, and an electronic copy, were donated to The British Library, London.
In 1946, Alan Turing wrote to Ashby suggesting he uses his ACE for his experiments instead of building a special machine. In 1948 Ashby made the Homeostat (Java applet simulation by Dr Horace Townsend). Ashby only made one reference to Turing in his notebooks in December 1954.
Ashby's Law of Requisite Variety (Ashby 1956), Variety absorbs Variety, defines the minimum number of states necessary for a controller to control a system of a given number of states. For example the number of bits necessary in a digital computer to produce a required description or model. In 1970 with Conant he produced the theorem "Every Good Regulator of a System must be a model of that System" (Int. J. Systems Sci., 1970. vol 1, No. 2 pp89-97). Stafford Beer applied Variety to found management cybernetics and the Viable System Model. Working independently Gregory Chaitin followed this with Algorithmic information theory.
From 1947 to 1959, Ashby was director of research at Barnwood House Hospital in Gloucester. From 1959 to 1960 he was Director of the Burden Neurological Institute. From 1960 to 1970 in the Department of Electrical Engineering at the University of Illinois at Urbana-Champaign. Ashby became a fellow of the Royal College of Psychiatry in 1971.