For those who haven’t had the dubious pleasure of reading Frank Herbert’s Dune, a mentat is a man (usually) whose mind, both by nature and by training, is almost like a machine in its ability to store and sort enormous amounts of data, and to make seemingly superhuman deductions and predictions thereby.

Herbert invented a universe for the mentat, but he wasn’t the first and won’t be the last to be drawn to the idea of a man whose power operates this way. Watching ‘Sherlock’ this week, it struck me that Holmes, as he’s presented in this series, is something like a mentat. He deliberately rids his mind of useless data (such as that the earth moves around the sun!) so as to store it with as much useful data as he can, and he has trained himself to sort and recall that data when he needs it. So too, as far as possible, he removes himself from any feelings about murder victims, because emotions get in the way of sorting data, and, therefore, solving a case.

He’s like Batman. (I mean the comic-book-Batman, not the movie-Batman). Granted, Batman doesn’t have extraordinary powers of recall or a mind-palace, but he is a detective, and data is his power. He keeps secret files about the weaknesses of his fellow members of the Justice League, just in case he ever needs to take them down. His Batcave is all databases and spyware. And he’s also incapable (deliberately or not; it’s hard to tell) of exercising normal human emotion or having a normal conversation with anyone – especially if that conversation involves the word, “Sorry.”

Mentats are obviously fascinating, and they exist in real life. The Dune, Sherlock, and Batman variations of the mentat are just imaginary glorifications of the real thing, made more interesting by drugs and martial arts (and both the mentats of Dune and Sherlock need drugs to do what they do.)

If you meet one, you’ll be impressed by the following features:

1) He loves power, but he doesn’t need the limelight. At times, he’ll even appear to prefer the shadows, as it were. He’s not looking to be a celebrity; he’s looking to be right.

2) He seems to know everything. Gradually, you’ll find that his mental hoard is of modern history, current affairs, and science, but somehow the absence of poetry and ancient history seems in his favour. He’s not a dork or a nerd; he’s a reader of living men.

3) He seems almost able to read souls at times. I knew a mentat once who nearly startled me out of my skin by telling me something about a young man that his family had taken years to work out, and no one outside the family had ever noticed, except him. And this was without ever having a real conversation with the man.

And this is all wonderful, but you need to understand this, too:

1) Most of his actions are calculated to an end. If a normal person introduces you to a friend, it’s because he thinks you’ll all get on and that will be nice. That is not why the mentat has introduced you, or why he’s put you both on the same committee, or sat you at the same table. He’s waiting for a result.

2) He does not love knowledge as a scholar loves knowledge, for its own sake. Everything he knows he knows because he suspects it will be in some way useful. Do not try to engage him in a conversation about poetry, or cognate languages, because he doesn’t care. If he’s listening to you, it’s to learn about you, to observe your gestures and expressions as you prattle on.

3) Don’t bother trying to get him to give up smoking. Mentats need drugs.

4) Deep in his innermost heart, he’s not convinced that it’s immoral to use people.

5) The worst thing that can happen to a mentat is to see a prediction or a calculation go wrong. This is why he will never, ever apologise. It’s not that he’s glad he hurt you; it’s that the prospect of admitting a mistake and feeling a moment’s human emotion about it is so appalling to him that it gives him something like a mental breakdown. And this is why he’ll walk away from you and cloak himself in silence, rather than talk (or listen). When it really matters, he needs his mentat-identity more than he needs his friends…or, at least, that’s what he’ll let you believe. Because who knows what he’s really thinking?