Characters in
Frank Herbert's
Dune novels, who are trained (generally from
birth) at performing
superhuman functions of
deductive reasoning and
logic. They were generally used to advise the leaders of the
Great Houses of the
Landsraad in
political matters, or for whatever purposes were necessary at the time.
Sapho juice was used to aid them in these deductive leaps.
Mentats were not "designed" to be emotionless; their minds functioned on such a level that they were able to compute data, unfettered by emotion. In a manner of speaking, when they were computing, emotions would be shoved to the side--it's part of the way they were trained.
Piter De Vries was an example of a "twisted Mentat", a mentat created by the Tleilaxu to serve as a human weapon; a person who is able to kill remorsely and cruelly, often used as assassins.
In David Lynch's movie version of Dune, Piter says the following little chant:
It is by will alone I set my mind in motion.
It is by the juice of Sapho that thought acquire speed;
That lips acquire stains; the stains become a warning.
It is by will alone I set my mind in motion.
He says this twice; it's interesting to note that the chant was not in any of the Dune novels, and that there is no reference to stains on the lips in the Dune books, either. But I'll tell you this - it burns into your memory.