On the definition of emotion:

Supposedly Mr. Spock, the Vulcan mastermind, didn’t have emotions […]. But Spock’s emotionlessness really just amounted to his being in control, not losing his head, coolly voicing unpleasant truths, and so on. He must have been driven by some motives or goals. Something must have kept Spock from spending his days calculating pi to a quadrillion digits or memorizing the Manhattan telephone directory. Something must have impelled him to explore strange new worlds, to seek out new civilizations, and to boldly go where no man had gone before. Presumably it was intellectual curiosity, a drive to set and solve problems, and solidarity with allies—emotions all. And what would Spock have done when faced with a predator or an invading Klingon? Do a headstand? Prove the four-color map theorem? Presumably a part of his brain quickly mobilized his faculties to scope out how to flee and to take steps to avoid the vulnerable predicament in the future. That is, he had fear. Spock may not have been impulsive or demonstrative, but he must have had drives that impelled him to deploy his intellect in pursuit of certain goals rather than others.

[…] Recall that intelligence is the pursuit of goals in the face of obstacles. Without goals, the very concept of intelligence is meaningless. To get into my locked apartment, I can force open a window, call the landlord, or try to reach the latch through the mail slot. Each of these goals is attained by a chain of subgoals. My fingers won’t reach the latch, so the subgoal is to find pliers. But my pliers are inside, so I set up a sub-subgoal of finding a store and buying new pliers. And so on. […]

But where does the topmost goal, the one that the rest of the program tries to attain, come from? For artificial intelligence systems, it comes from the programmer. The programmer designs it to diagnose soybean diseases or predict the next day’s Dow Jones Industrial Average. For organisms, it comes from natural selection. The brain strives to put its owner in circumstances like those that caused its ancestors to reproduce. (The brain’s goal is not reproduction itself; animals don’t know the facts of life, and people who do know them are happy to subvert them, such as when they use contraception.) The goals installed in Homo sapiens, that problem-solving, social species, are not just the Four Fs. High on the list are understanding the environment and securing the cooperation of others.

And here is the key to why we have emotions. An animal cannot pursue all its goals at once. If an animal is both hungry and thirsty, it should not stand halfway between a berry bush and a lake[…]. Nor should it nibble a berry, walk over and take a sip from the lake, walk back to nibble another berry, and so on. The animal must commit its body to one goal at a time, and the goals have to be matched with the best moments for achieving them. Ecclesiastes says that to every thing there is a season, a time to every purpose under heaven: a time to weep, a time to laugh; a time to love, and a time to hate. Different goals are appropriate when a lion has you in its sights, when your child shows up in tears, or when a rival calls you an idiot in public.

The emotions are mechanisms that set the brain’s highest-level goals. Once triggered by a propitious moment, an emotion triggers the cascade of subgoals and sub-subgoals that we call thinking and acting. Because the goals and means are woven into a multiply nested control structure of subgoals within subgoals within subgoals, no sharp line divides thinking from feeling, nor does thinking inevitably precedes feeling or vice versa […]. For example, fear is triggered by a signal of impending harm like a predator, a clifftop, or a spoken threat. It lights up the short-term goal of fleeing, subduing, or deflecting the danger, and gives the goal high priority, which we experience as a sense of urgency. It also lights up the longer-term goals of avoiding the hazard in the future and remembering how we got out of it this time, triggered by the state we experience as relief. Most artificial intelligence researchers believe that freely behaving robots (as opposed to the ones bolted to the side of an assembly line) will have to be programmed with something like emotions merely for them to know at every moment what to do next. (Whether the robots would be sentient of these emotions is another question, as we saw in Chapter 2.)

Fear also presses a button that readies the body for action, the so-called fight-or-flight response. (The nickname is misleading because the response prepares us for any time-sensitive action, such as grabbing a baby who is crawling toward the top of a stairwell.) The heart thumps to send blood to the muscles. Blood is rerouted from the gut and skin, leaving butterflies and clamminess. Rapid breathing takes in oxygen. Adrenaline releases fuel from the liver and helps the blood to clot. And it gives our face that universal deer-in-the-headlights look.

Each human emotion mobilizes the mind and body to meet one of the challenges of living and reproducing in the cognitive niche. Some challenges are posed by physical things, and the emotions that deal with them, like disgust, fear, and appreciation of natural beauty, work in straightforward ways. Others are posed by people. The problem in dealing with people is that people can deal back. The emotions that evolved in response to other people’s emotions, like anger, gratitude, shame and romantic love, are played on a complicated chessboard, and they spawn the passion and intrigue that misleads the Romantic.