The Butlerian Jihad was an epic war against AI beings that had enslaved humanity. After the human victory, the humans outlawed non-human computation. Hence various guilds of human computers (e.g. the mentats) formed. 

From [Wikipedia][1]

> The Butlerian Jihad is an event in the back-story of Frank Herbert's fictional Dune universe. Occurring over 10,000 years before the events chronicled in his 1965 novel Dune, this jihad leads to the outlawing of certain technologies, primarily "thinking machines," a collective term for computers and artificial intelligence of any kind. This prohibition is a key influence on the nature of Herbert's fictional setting.

From the glossary in Dune

> JIHAD, BUTLERIAN: (see also Great Revolt) — the crusade against computers, thinking machines, and conscious robots begun in 201 B.G. and concluded in 108 B.G. Its chief commandment remains in the O.C. Bible as "Thou shalt not make a machine in the likeness of a human mind."


  [1]: http://en.wikipedia.org/wiki/Butlerian_Jihad