The Great Mental Models summary

Imagine all of our practical human knowledge as forming a vast and varied landscape, encompassing everything from knowing how to cook to knowing how to invest in the stock market to knowing how to climb Mount Everest. To navigate most of this terrain, you'll need a map – whether it takes the form of a cookbook, an investment manual, or a mountaineering guide.

Other places, though, will be so familiar to you that a map becomes unnecessary. These places are the skills you’ve mastered through years of study, practice, and hard work – and they all fall within your circle of competence.

When you focus on your circle of competence, you’re playing to your strengths. But there’s more to success than mere competency. You also need creativity. It might sound like a cliché, but it’s true: you have to think outside the box. Otherwise, you’ll end up doing the same old thing that everyone else is doing. But how do you break away from the pack?

The trick lies in digging deep. For any subject or body of knowledge, if you drill down deep enough, you’ll eventually find what you’re looking for.

Imagine you’re an advertising executive working for the American Tobacco Company in the 1920s. The company has noticed that most smokers are men, and you’ve been tasked with the following objective: sell more Lucky Strike cigarettes to women. Putting aside any moral scruples, how would you achieve this goal?

That was the question once faced by Edward Bernays – the Austrian-American pioneer of the fields of public relations and propaganda. He answered it by implementing the next mental model we’re going to look at. It’s a nifty little trick called inversion, which involves taking the normal approach to problem-solving and turning it upside down.

Now that you’ve learned first-principle reasoning and the technique of inversion, there’s no limit to how many creative ideas you can come up with. Hopefully they’ll be more like growing artificial meat than getting people to smoke! But before you try them out in the real world, you’ll probably want to simulate them in your imagination first.

The key message here is: Use thought experiments to test out your ideas and clarify your thinking.

Imagine you’ve just won the lottery! What’s the first thing you’d do? Let’s say you buy a bigger house. Great – but now you have to consider how this decision will change your life. For example, you might have to spend more time cleaning. Either that, or have to hire a cleaner.

Either way, you’ve now gone from thinking about the consequences of something, as you do when conducting a thought experiment, to thinking about the consequences of the consequences. This is called second-order thinking.

Second-order consequences can often come as an unpleasant surprise. But that doesn’t mean you should get too carried away guarding against them. With any given action, you can imagine all sorts of horrible things happening. If you fixate on them, though, you may become paralyzed with indecision, or overreact by drawing extreme conclusions.

Call it the slippery slope effect. One moment, you’re thinking about all the potential negative consequences of drinking alcohol, such as alcoholism. Next thing you know, you’re advocating for prohibition! That would be an overreaction. Sure, drinking alcohol can lead to alcoholism, but it doesn’t always. To temper our fears and assess the potential consequences of an action rationally, we need to think about their probabilities.

Imagine you wake up one morning feeling sick. Naturally, you go online and look up your symptoms – which, you discover, could either be caused by the flu or by Ebola. Which explanation should you believe? The answer is important. It’s the difference between going back to bed or going into quarantine.

In a situation like this, you’re no longer just weighing an individual piece of evidence, as you do with probabilistic thinking. Now you need to decide between two overall explanations for the evidence you have at hand.

From deadly illnesses to mere tardiness, Occam’s razor provides a great way to decide between competing explanations of things. But the vagaries of human behavior are varied, and sometimes we’ll need another type of blade. This brings us to our final mental model: Hanlon’s razor.

You can think of Hanlon’s razor as a close cousin to Occam’s razor. Like its more famous relative, Hanlon’s razor starts by reminding us that the simpler explanation – all other things being equal – tends to be the safest bet. But then it adds a special twist.