System 1 is the part of the brain that handles the simple things: sensory input, automatic and unimportant decisions, casual social interactions, and other inbound signals that can be processed rapidly and rather easily.

System 1 is rapid and instinctive. It does not involve what we usually associate with ‘thinking’. When you duck unexpectedly because a ball is thrown at you, or get nervous when you look down from the rooftop of a skyscraper, or smile when you see a funny meme on 9GAG, you are using System 1. These activities are associated with the most primitive parts of the brain.

System 2 is the higher-order, logical part of the brain. It’s the part that thinks at the speed of the voice in your head. It brings processing power for decisions and problems that require deeper thought.

System 2 is more deliberate and self-conscious. We use it while learning to drive, when deciding which flight to take to London, and whether to become an engineer or a doctor.

All Indians have a System 1 reaction to a temperature given in Celsius but have to use their System 2 to process a temperature given in Fahrenheit; for Americans, the opposite is true. People speak their native languages using System 1 and tend to struggle to speak a new language using System 2. Being truly bilingual means that you speak two languages using System 1. Professional chess players’ System 1 allows them to size up complex situations rapidly and to respond with both amazing accuracy and exceptional speed.

System 1 is your gut reaction and the System 2 is your conscious thought.

System 1 can be trained with lots of repetition — but it takes a lot of time and effort. One reason why newbie drivers are such risky on the road is that their System 1 has not had much practice, and System 2 is much slower.

The workings of the human brain can be befuddling. Beethoven wrote his incredible ninth symphony while he was deaf, but he often misplaced his house keys. How can we be so ingenious at some tasks and so clueless at others? If you think about all the things the brain is constantly handling — it’s not just impressive, it’s insane. But you’re able to do it because the part of your brain that you’re aware of, System 2, is always outsourcing the bulk of the work to System 1.

The fox knows many things but the hedgehog knows one big thing .

Decades ago, the philosopher Isaiah Berlin wrote an essay that compared the styles of thinking of great authors through the ages. To organise his observations, he drew on a scrap of 2,500-year-old Greek poetry attributed to the warrior-poet Archilochus: “The fox knows many things but the hedgehog knows one big thing.”

Think of System 1 as childlike. Just like 5-year-olds view everything in terms of cause and effect — and has absolute certainty about the things they know — this part of the brain will either believe something with great conviction or not. Nothing in between. It’s like the hedgehog.

Think of System 2 as a beleaguered, overworked but very intelligent manager. System 1 is the army of interns that she’s hired to solve all the simple problems she can delegate — even though they mess some of them up.

System 1 has no time for, ‘This thing has a 30% probability of being true.’ That’s System 2. And the thing about System 2 is that it’s always looking for evidence that something isn’t right or isn’t to be believed. It’s nuanced. It’s a sceptic. It’s like the fox.