A Mentat is a fictional profession or discipline in Frank Herbert's Dune universe. Mentats are humans trained to mimic computers: human minds developed to staggering heights of cognitive and analytical ability.
As established by Frank Herbert, following the defeat of the thinking machines by humanity in the Butlerian Jihad, it was forbidden to create sentient machines:
Thou shalt not make a machine in the likeness of a human mind. — Orange Catholic Bible
The Mentat discipline was developed as a replacement for computerized calculation, just as the Bene Gesserit and the Spacing Guild took on functions previously provided by thinking machines. For thousands of years, society considered Mentats the embodiment of logic and reason.
Unlike computers, however, Mentats are not simply human calculators writ large. 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, that can extract the essential patterns or logic of 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, filling roles as menial as archivists and clerks, or as grand as advisors to Emperors.
A Mentat's capabilities can be greatly increased by taking sapho juice, but using it leads to addiction. Sapho is extracted from the root of the Ecaz tree and its use doubles or even triples the Mentat's immense processing power. However, 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. The ability to be a Mentat is a talent limited to a small number of people; thus, those with the discipline are prized in whatever role they choose. There are also varying degrees of ability. In Dune, Thufir Hawat is considered to be one of the best mentats known at the time. Paul Atreides is a Mentat duke (and later emperor) in Dune, and in Heretics of Dune and Chapterhouse Dune, Miles Teg is a Mentat bashar (or general) and the most formidable military commander in the known universe. In Dune Messiah, Duncan Idaho is recreated as a ghola, and trained as both a Mentat and as a Zensunni philosopher; he then serves Paul as the House Weapons Master. In Children of Dune, 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 the 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 (de Vries is, among other things, a sadist and acts as torturer for the Baron). In Heretics of Dune the Duncan ghola supplied to the Bene Gesserit by the Tleilaxu has been altered in many subtle and hidden ways by the Tleilaxu, including the addition of submerged Mentat abilities.
In God Emperor of Dune, Leto II outlaws the order and crushes any renegade training schools he finds. Unlike his execution of historians for the purpose of controlling his legacy and symbolism, Leto never explicitly mentions 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.
 Origins The origin of the first Mentat is explored in the Legends of Dune prequels written by Kevin J. Anderson and Brian Herbert. During the Butlerian Jihad, thinking machine leader Omnius sees humans as animals, but the independent robot Erasmus argues that any human can become brilliant. Omnius picks a nine-year-old, blonde-haired boy who appears to be the wildest and most unkempt of all, and challenges Erasmus to prove his theory. Erasmus calls the boy Gilbertus Albans, thinking that this sounds like a smart human's name. After initially failing to make progress by using a system of benevolence and rewards, he switches to a system of strict supervision and punishment, and the method works. By emulating Erasmus, whom he has come to consider his father, Albans becomes the first to display computer-like cognitive and calculation capacity on the level of thinking machines.
To his own surprise, Erasmus too develops an attachment to the boy; when Gilbertus is about 20 years old, Erasmus performs a life-extension procedure on him without permission from Omnius. Because of Gilbertus' remarkable memory-organizational ability and capacity for logical thinking, Erasmus nicknames him "Mentat," created from the words mentor, mentee and mentation. During the Battle of Corrin, Erasmus deactivates an explosive trip mechanism in the thinking machine defenses in order to save Gilbertus, thereby dooming the entire machine empire. The robot says:
Perhaps when all the thinking machines are gone, you can teach your fellow humans how to think efficiently. Then all my work will not have been for nothing. — Dune: The Battle of Corrin
The Mentat Mantra The David Lynch Dune film includes a scene in which Piter De Vries recites the following upon drinking sapho juice:
It is by will alone I set my mind in motion. It is by the juice of sapho that thoughts acquire speed, the lips acquire stains, the stains become a warning. It is by will alone I set my mind in motion. This mantra is a creation of Lynch and does not appear in the books, though in Dune there is mention that Thufir Hawat has cranberry-stained lips from sapho juice.
The same mantra is also recited by the Harkonnen Mentat Hayt in the beginning of the House's campaign in the Dune 2000 video game before briefing the player on the first mission.
"Mentats" is also the name of a drug in 1997 computer game Fallout and its 1998 sequel Fallout 2, which temporarily enhanced the user's mental attributes