Two minds, one head.

Daniel Kahneman was a psychologist who won the Nobel Prize in economics, which tells you how far his ideas travelled. Over decades of experiments, mostly with his collaborator Amos Tversky, he took apart the flattering assumption at the centre of modern economics and much of modern life, the belief that human beings are rational agents who weigh the evidence and choose well.

His book gives the finding a memorable shape. We think with two systems. System 1 is fast, automatic and emotional, the part that jumps to conclusions, reads a face, finishes the phrase. System 2 is slow, deliberate and effortful, the part that actually does the arithmetic. We identify with System 2, the careful reasoner. But most of the time System 1 is quietly running the show, and it is riddled with predictable errors.

Kahneman catalogued those errors, the biases and shortcuts that steer us wrong without our noticing. We judge risk by how easily an example comes to mind. We are anchored by the first number we hear. We feel a loss about twice as hard as an equal gain. None of these are signs of stupidity. They are the standard equipment of a normal mind, and knowing about them barely helps, which is the book's most humbling lesson.

What makes the work matter beyond psychology is the honesty of it. Kahneman spent his career, including the celebrated partnership at its heart, showing his own field where it had been fooling itself, and he never exempted himself from the findings. He wrote openly about how his own intuitions failed the tests he designed.

For anyone who makes decisions, which is everyone, the book is a kind of humility engine. It does not make you smarter. It makes you suspicious of the confident voice in your head, and teaches you to build in the friction, the second opinion, the slept-on decision, that catches the fast mind before it commits you.

Next time you feel certain, slow down and ask which of your two minds is doing the talking.