Overview
In Herbert's fiction, following the defeat of the thinking machines by humanity in the Butlerian Jihad, it is forbidden to create sentient machines. The Mentat discipline is developed as a replacement for computerized calculation, just as the Bene Gesserit and the Spacing Guild take on functions previously performed by thinking machines. For thousands of years, society considers Mentats the embodiment of logic and reason.
Unlike computers, however, Mentats are not simply calculators. Instead, the exceptional cognitive abilities of memory and perception are the foundations for supra-logical hypothesizing. Mentats are able to sift large volumes of data and devise concise analyses in a process that goes far beyond logical deduction: Mentats cultivate "the naïve mind", the mind without preconception or prejudice, so as to extract essential patterns or logic from data and deliver useful conclusions with varying degrees of certainty. They are not limited to formulating syllogisms; they are the supreme counselors of the Dune universe, as menial as an economist or a historian who might advise leadership, or as grand as advisor to the Emperor. So the Mentat is not a purely "left brain" type of intelligence with only a simplistic-autistic cognition, they are profoundly aware of nuance, subtlety and the kind of "sensitivity to initial conditions" which is studied in Chaos theory. Additionally, they take in information through visual as well as other media and their calculations are delivered not as numerical probabilities but as flowing paths, subject to new variations through the influence of new factors. In all of these ways the mentats' resemble less the literal/clerical/linear autistic mind and more the global, variable and visual mind of the dyslexic.
A Mentat's capabilities can be greatly increased by taking sapho juice, but using it leads to addiction. Sapho is extracted from roots found on Ecaz, and its use doubles or even triples the Mentat's already immense processing power. Repeated use leaves a permanent "cranberry-colored stain" on the user's lips.
Being a Mentat can be a profession in itself, or the discipline can be added to other roles. Few people have the ability to be a Mentat; thus, those with the discipline are prized in whatever role they choose. There are also varying degrees of ability. In Dune (1965), Thufir Hawat is considered to be one of the best Mentats of his time. Paul Atreides is a Mentat duke (and later emperor) in Dune, and in Heretics of Dune (1984) and Chapterhouse: Dune (1985), Miles Teg is a Mentat bashar (or general) and the most formidable military commander in the known universe. In Dune Messiah (1969), Duncan Idaho is recreated as a ghola and trained as both a Mentat and a Zensunni philosopher. In Children of Dune (1976), he marries the Regent, Paul's sister Alia, but is then instrumental in protecting the royal heirs from her, demonstrating that Mentats are rarely simply disinterested observers and counselors.
It is also possible to vary the discipline: In Dune, the Tleilaxu create "twisted Mentats" for Baron Vladimir Harkonnen (Piter De Vries being the most notable example). These are sociopathic Mentats free from the usual constraints of human morality or instinctual considerations (Piter is, among other things, a sadist, and acts as torturer for the Baron). In Heretics of Dune the Tleilaxu supply the Bene Gesserit with a Duncan ghola they have altered in many subtle and hidden ways, including the addition of submerged Mentat abilities.
In God Emperor of Dune (1981), Leto II outlaws the order and crushes any renegade training schools he finds. Unlike his execution of historians, done to control his legacy and symbolism, Leto never says why he suppresses Mentats, leaving the reader to conclude that it simply plays a role in his larger purpose of oppressing humanity in order to create his Golden Path. The art is not eliminated, however, surviving through underground schools; notably, the Bene Gesserit preserve the art, assuming that Leto knows of this (through his prescience) and approves.
Later, in Chapterhouse Dune, the Duncan ghola speaks to the Bene Gesserit Reverend Mother Bellonda and recalls his past incarnation from the time of Leto II:
I went to Wallach IX at the Tyrant's command ... My orders were to suppress the Mentat school you thought you had hidden there ... I, a Mentat, forced to suppress a school that trained people the way I was trained. I knew why he ordered it, of course, and so do you.
Bellonda retrieves the following information from Other Memory:
Order of Mentats, founded by Gilbertus Albans; temporary sanctuary with Bene Tleilax who hoped to incorporate them into Tleilaxu hegemony; spread into uncounted "seed schools"; suppressed by Leto II because they formed a nucleus of independent opposition; spread into the Scattering after the Famine.
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