George Dixon would have called it a "copper's nose". The ability to sum up a situation in seconds. Knowing when to go easy and when to wield the big stick - a natural talent for understanding the workings of the miscreant mind.
At Dock Green such an attribute went hand-in-hand with a firm- but-fair attitude, a brave heart and a pair of shiny boots to make a first-rate officer. If you'd asked George directly, however, he'd have told you he had little time for psychology.
But times change. Of the hundreds of police employees who graduated with the Open University last year, more than one third studied psychology or social sciences.
"Psychology helps you to understand why the people you deal with behave as they do," says Gary Marsh, 43, a sergeant with Wiltshire Police, who progressed to a psychology degree after "a couple of bad experiences" prompted him to train as a counsellor to help fellow officers.
"Police officers have always used psychology, often without realising. They have gained convictions after forming perceptions and following them through to the nth degree. That's why there are so many miscarriages of justice stemming from the Seventies and Eighties, but not from the Nineties. Police work has got better and understanding those perceptions has improved."
But don't be misled - it may suggest a softly, softly approach, but the police is a task force. There'll be little time for group hugs.
"You always have to remember you're not a social worker or a psychologist," says Sgt Marsh, who began his police career patrolling a quiet patch "like something out of Heartbeat" in Salisbury.
"Sometimes you have to use the full weight of the law. You're still the big stick, the hammer to crack a nut.
"It's a line you have to learn to draw. You can advise what to do, and sometimes psychology does help you to understand some of the reasons that these things are happening, but you're paid to be a police officer, acting in the interests of the community. That's always your priority."
So while an understanding of psychology can certainly help officers to see the roots of behaviour, no criminal will be let off too leniently as a result. Psychology may make people victims of circumstance, but it does not, in police eyes, excuse criminals whose evidence of guilt is more than circumstantial.
"A thief is still a thief, a burglary is still a burglary, a fight is still a fight," says Sgt Marsh. "And while there are an awful lot of good people about - particularly good young people - a greater minority of the public is more vindictive than it used to be.
"Another advantage of psychology is that it helps you to understand yourself. It can really help you when aggravation builds up - complaints at work, family pressures because of the hours you work and the job you do. …