You use two systems for thinking. System 1 is fast, automatic and unconscious (when you see 2 + 2, your brain automatically says 4). System 2 is slow, deliberate and taxing on your mental resources (if you see 22 x 17, you probably need to think to find the answer). System 1 makes thousands of unconscious decisions everyday – and most of them are fine. Sometimes, however, System 1 makes mistakes, which can lead to cognitive biases.