A mentat was a profession or discipline that was developed as a replacement to the thinking machines following the Butlerian Jihad that banned the creation of machines in the human mind's image, just as the Spacing Guild and the Bene Gesserit took up similar functions. The term 'mentat' was first coined by Erasmus, created from the words 'mentor' and 'mentee'.
The first mentat
Gilbertus Albans displayed the first cognitive thinking and computer-like calculation capacity necessary for a Mentat.
In the aftermath of the Butlerian Jihad, computers and other forms of thinking technology were made obsolete due to the OCB's commandment against thinking machines, "Thou shalt not make a machine in the likeness of a human mind", and, at the same time, the anti-AI laws had been placed in effect; the punishment of owning such AI device or any kind being sentenced to immediate death. Human computers, also known as mentats, trained as replacements to the mechanical thinking technology, just as the all-female Bene Gesserit and the Guild of Navigators took up the same functions that had been previously covered by the Thinking Machines prior to the Jihad which banned all thinking technology.
Impact of God-Emperor rule
Along with the Spacing Guild and the Bene Gesserit, the rise of Leto Atreides II to the title of God Emperor brought with it new problems for the entire mentat order. Leto's extremely long life, coupled with his goal to realize The Golden Path in order to ensure humanity's survival, saw him horde the Spice Melange without concern for short-term problems.
During the 3,500-year rule, the mentat order was outlawed by God Emperor Leto II, and any renegade training schools crushed. The art of being a mentat was not eliminated, however, survived through underground schools; the Bene Gesserit preserved the art, assuming that Leto knew of this and allowed.
Use of Mentats
Mentats were used extensively by the Great Houses, primarily as political advisors before the rise of God Emperor Leto Atreides II to the throne. Thanks to their vast memories and ability to organize huge amounts of data they often provided valuable insights that would otherwise be lost. Limited mentat training was also used to augment individuals destined for other political or military roles.
The role of mentat required a certain degree of mental ability, and strict mental conditioning. Though not all mentats were of equal ability, all were typically able to guarantee employment with one of the power brokers of the Known Universe.
Unlike computers, however, mentats was not simple human calculators. Instead, the exceptional cognitive abilities of memory and perception were the foundations for supra-logical hypothesizing. Mentats were able to gather large amounts 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, with varying degrees of certainty, useful conclusions. Indeed, this ability saw many military commanders and political leaders undergo some form of mentat training. Examples of this include Paul Atreides, Miles Teg, and most of the Duncan Idaho gholas. A few individuals, such as Thufir Hawat, had sufficient innate mentat abilities to be an exclusive advisor to House Atreides.
The ability to become a mentat was a talent limited to a small number of people; thus, those with the discipline were prized in whatever role they choose. There are also varying degrees of ability. 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 turns the user's lips a shade of red.
Mentats were required to operate within an ethical framework, which for some purposes was not desired. Thus, the Bene Tleilax were known to condition some mentats to be 'twisted'. These individuals, such as Piter de Vries, were able to compute trends free of any ethical restraints or limitations.
The Mentat Mantra
"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."