In Emotional: How Feelings Shape Our Thinking (Pantheon, Jan.), physicist Mlodinow lays out recent discoveries about the role emotions play in everyday life.

You call new scientific discoveries in emotions a revolution. What do you mean by that?

We’ve had a standard theory of emotion since Darwin, who wondered what the purpose, and the origin, of emotion was. He came up with a neat, simple description. Since then, people have developed that theory, adding that there are certain very well-defined centers in the brain for each emotion and that they’re universal among all cultures. And so they developed a very reasonable-sounding standard theory that fit very well with our intuition about what emotion is. In the last 10 years, they’ve started to really knock down that old theory. For example, there are actually dozens of emotions, and people don’t believe anymore that some are more fundamental than others.

Is distinguishing between emotions and thought useful?

I think that some would say there isn’t any value. I still believe that there is, but you can’t make a sharp distinction. The idea is that rational thought is equated with logic. It’s harder to define emotion. In fact, I think I even mentioned in the book that there’s not even a good, universally accepted definition of emotion that people have come up with. It’s that open. But the one that I like the best is the one of emotion as being a functional state of the brain.

How does emotion function, then?

One of my book’s themes is that the human brain—or any animal brain—is an information processing machine. And what it does is it tries to predict the future to achieve your goals, and there’s a whole variety of answers you could get. And that’s where emotions come in, because they guide you toward processing that data in a certain way. So, if you are hungry, then you’ll process the sight of a candy bar in one way, and you might not notice it if you’re not hungry. There are a lot of studies about how people are blind to a lot of what’s in their visual field. You might be hungry, and then you’re walking down a dark street and you think someone is following you, suddenly, you’re not hungry anymore, because now you’re interpreting everything in your environment and making decisions about your actions not based on your hunger, but on not getting mugged. That’s what emotions are, a mode of processing that your brain can be guided by.