People are often told to be rational instead of emotional. The view that emotions conflict with reason goes back at least to Plato, who said that the intellect needs to control passions as a charioteer controls a horse. There are indeed many ways in which emotional states can interfere with making good inferences, ranging from psychiatric problems such as mania and depression to more everyday afflictions such as wishful thinking, motivated inference, weakness of will, and self-deception. Understanding emotional brain processes can help us to deal with these problems, but it can also help us to appreciate how emotions are essential for effective thinking in all domains, from practical decision making to scientific discovery.