This article is an excerpt from the Shortform summary of "Thinking, Fast and Slow" by Daniel Kahneman. Shortform has the world's best summaries of books you should be reading.
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In Thinking, Fast and Slow, Daniel Kahneman defines System 1 and System 2 thinking—the two different ways you think and make decisions. These are the titular “fast” and “slow” methods of thinking, each with its own benefits and drawbacks.
In this article, we’ll start by describing the quick, largely subconscious System 1, then move on to the slower and more rational System 2. We’ll also discuss why Kahneman believes that each system has its own fundamental flaw.
Originally Published: November 5, 2019
Last Updated: December 15, 2025
System 1: Thinking Fast
We’ll explain System 1 and System 2 thinking in-depth. Kahneman explains that System 1 operates automatically and quickly, with little or no conscious effort and no sense of voluntary control. This system gives rise to your feelings and intuitions, and it suggests courses of action for your conscious mind to consider. In short, System 1 is Kahneman’s collective term for all the ways you think fast.
Some examples of System 1 thinking include your ability to tell that one object is farther away than another, to read and understand simple sentences, to detect the emotions in someone else’s voice, and to perform simple or habitual actions like driving a car on an empty road.
(Shortform note: System 1 thinking gives rise to what Malcolm Gladwell (Blink) calls unconscious thinking, better known as intuition or a “gut feeling.” Gladwell believes unconscious thinking is often as effective as conscious, rational thinking, with the added benefits of happening more quickly and being less likely to get derailed by stress or anxiety. He explains that your subconscious mind naturally takes in a huge amount of information—much more than you consciously process—filters out what’s irrelevant, and returns the best answer to your current problem.)
Kahneman further explains that System 1 operates through association: It rapidly connects what you see or experience to related concepts and patterns stored in your memory. By doing so, this mental system can generate impressions and judgments nearly instantly.
For instance, if you see someone frowning, System 1 thinking instantly associates that input with related concepts like anger, threat, and negativity. As a result, within milliseconds of seeing that expression, you’re primed to defend yourself, escape, or defuse the situation.
(Shortform note: Associative thinking like Kahneman describes here isn’t just fast, it’s also creative—in fact, in The Innovator’s DNA, Hal Gregersen, Jeff Dyer, and Clayton Christensen argue that associative thinking is the very foundation of creativity. They explain that creative ideas and innovations usually aren’t completely new concepts, but rather come from people connecting concepts in ways that nobody had connected them before.)
System 1 Is Inaccurate
Since you can’t stop to consciously think about everything you see, hear, or otherwise experience, you need System 1 to function in your everyday interactions. However, Kahneman adds that this system is impulsive and imprecise.
To continue the previous example, you might see someone frowning and, because of System 1 thinking, immediately assume the person is angry and that you’re in danger. However, they could be frowning for any number of reasons: Perhaps they’re simply thinking over a difficult problem, and the expression has nothing to do with you at all.
Trick questions provide perfect demonstrations of System 1’s fallibility. For instance: According to the Old Testament, how many of each type of animal did Moses take on the ark?
You most likely thought of the number two immediately. However, if you think carefully, you’ll realize the answer is actually zero—it was Noah who took animals on the ark, not Moses. Your associative System 1 thinking saw the words “animal” and “ark” and answered the question it thought was being asked, rather than the actual question.
| System 1 Reliance Is an Evolutionary Holdover If System 1 thinking is so unreliable, why do we depend on it so much? In large part, the answer lies in our evolutionary history: For our ancient ancestors, making fast decisions was much more important for survival than making perfectly accurate decisions. For example, suppose two ancient humans saw a pair of eyes staring out of a nearby bush—the person who immediately runs away is more likely to survive than the one who makes sure it really is a predator before starting to run. Therefore, the first person survives and passes on their quick-thinking genes, while the second person doesn’t. The next generation then tends more heavily toward quick thinking than careful decisions. As for the reason we haven’t continued to evolve and become better suited to our lower-risk modern environment—why we haven’t gradually traded speed for accuracy in our thinking—it’s because we simply haven’t needed to in order to survive. While evolution is commonly framed as survival of the fittest, some biologists argue that it would be more appropriate to call it survival of the adequate. In other words, evolution doesn’t try to create some hypothetical perfect species. Instead, it’s simply the process of “good enough” organisms creating more “good enough” organisms. |
System 2: Thinking Slow
Your other method of thinking is what Kahneman refers to as System 2. This system allocates your attention to the effortful mental activities that demand it. Some common examples of tasks that require System 2 thinking include maintaining your focus, making difficult choices, and performing complex calculations. This mental system also converts your intuitions into beliefs and your impulses into voluntary actions.
The author adds that everything you encounter will first go through the much faster System 1. System 2 only activates when there’s something you can’t resolve with faster, associative thinking. For instance, driving your car on an empty road is generally a System 1 task—it’s simple and familiar enough that you can probably do it without conscious thought. However, if you need to parallel park in a tight space, your mind will activate System 2 so you can make careful adjustments to avoid the cars around you.
| System 1 Can Also Back Up System 2 Kahneman describes System 2 as a backup system that kicks in when System 1 can’t resolve an issue. However, sometimes it’s the other way around, and you need System 1 to solve problems that System 2 gets stuck on. In A Mind For Numbers, Barbara Oakley describes “diffuse-mode” and “focus-mode” thinking—two methods of thought that resemble Kahneman’s two systems. In diffuse-mode thinking, which roughly corresponds to System 1, your thoughts traverse longer neural pathways between more diverse concepts. This may lead you to connect distantly related ideas in ways that you’d have never thought of while using active focus-mode thinking (similar to Kahneman’s System 2). That’s why answers sometimes come to you in a sudden flash of insight after you’ve stopped thinking about a problem. This also means that, when you’re stuck on a difficult issue, sometimes the best thing to do is take a break and let your mind relax—let System 1 take over for a while and see what it comes up with. |
System 2 Is Lazy
Kahenman writes that your brain defaults to System 1 not only because it’s faster, but also because it uses less mental energy, and your brain instinctively tries to reserve your mental resources however it can. In simpler terms, System 2 is lazy, so it wants to let System 1 handle as much of the thinking as possible.
As a result, you often won’t question the impulses and intuitions that System 1 generates, as rethinking your System 1’s decisions would use energy your brain would rather not expend. This problem gets worse when you’re tired or stressed—your limited mental resources are already strained, which makes you even less able to devote conscious effort to solving problems and making decisions.
(Shortform note: To compound the problem of limited mental resources, modern life practically guarantees that you’re always mentally exhausted. In Willpower Doesn’t Work, organizational psychologist Benjamin Hardy explains how you’re barraged by decisions and temptations every single day. Each decision drains your mental reserves, leaving you less able to make good decisions—and more likely to default to whatever’s easiest—as the day goes on.)
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- Why we get easily fooled when we're stressed and preoccupied
- Why we tend to overestimate the likelihood of good things happening (like the lottery)
- How to protect yourself from making bad decisions and from scam artists
