William Ross Ashby (6 September 1903 – 15 November 1972) was an English psychiatrist and a pioneer in cybernetics, the study of the science of communications and automatic control systems in both machines and living things.

[1]: 93  These "missionary works" along with his technical contributions made Ashby "the major theoretician of cybernetics after Wiener".

[2][3]: 28 William Ross Ashby was born in 1903 in London, where his father was working at an advertising agency.

[4] Despite being widely influential within cybernetics, systems theory and, more recently, complex systems, Ashby is not as well known as many of the notable scientists his work influenced, including Herbert A. Simon, Norbert Wiener, Ludwig von Bertalanffy, Stafford Beer, Stanley Milgram, and Stuart Kauffman.

In 2003, these journals were given to The British Library, London, and in 2008, they were made available online as The W. Ross Ashby Digital Archive.

[7] Ashby initially considered his theorizing a private hobby, and his later decision to begin publishing his work caused him some distress.

[1]: 98  The machine used a simple mechanical process to return to equilibrium states after disturbances at its input.

Norbert Wiener, describing the appearance of purposeful behavior in the Homeostat's random search for equilibrium, called it "one of the great philosophical contributions of the present day".

The book gave accounts of homeostasis, adaptation, memory and foresight in living organisms in Ashby's determinist, mechanist terms.

[2] Ashby's 1964 paper Constraint Analysis of Many-Dimensional Relations began the study of reconstructability analysis, a multivariate systems modeling methodology based on set theory and information theory, which would later be developed by Klaus Krippendorff, George Klir, and others.

Mathematically, the law is a statement about how "in a two-person game the variety possible is determined by the number of possible choices open to the two players".

[19] On 4–6 March 2004, a W. Ross Ashby centenary conference was held at the University of Illinois at Urbana–Champaign to mark the 100th anniversary of his birth.

[20] In February 2009, a special issue of the International Journal of General Systems was specifically devoted to Ashby and his work, containing papers from leading scholars such as Klaus Krippendorff, Stuart Umpleby and Kevin Warwick.

Ashby's work on the law of requisite variety has influenced scholars within the field of management studies.