The ability to remember motor acts like changing lanes is called procedural memory, and it is a type of implicit memory—meaning that your brain holds knowledge of something that your mind cannot explicitly access. Riding a bike, tying your shoes, typing on a keyboard, and steering your car into a parking space while speaking on your cell phone are examples of this. You execute these actions easily but without knowing the details of how you do it. You would be totally unable to describe the perfectly timed choreography with which your muscles contract and relax as you navigate around other people in a cafeteria while holding a tray, yet you have no trouble doing it. This is the gap between what your brain can do and what you can tap into consciously.

The concept of implicit memory has a rich, if little-known, tradition. By the early 1600s, René Descartes had already begun to suspect that although experience with the world is stored in memory, not all memory is accessible. The concept was rekindled in the late 1800s by the psychologist Hermann Ebbinghaus, who wrote that “most of these experiences remain concealed from consciousness and yet produce an effect which is significant and which authenticates their previous existence.”

To the extent that consciousness is useful, it is useful in small quantities, and for very particular kinds of tasks. It’s easy to understand why you would not want to be consciously aware of the intricacies of your muscle movement, but this can be less intuitive when applied to your perceptions, thoughts, and beliefs, which are also final products of the activity of billions of nerve cells. We turn to these now.

Chicken Sexers and Plane Spotters
When chicken hatchlings are born, large commercial hatcheries usually set about dividing them into males and females, and the practice of distinguishing gender is known as chick sexing. Sexing is necessary because the two genders receive different feeding programs: one for the females, which will eventually produce eggs, and another for the males, which are typically destined to be disposed of because of their uselessness in the commerce of producing eggs; only a few males are kept and fattened for meat. So the job of the chick sexer is to pick up each hatchling and quickly determine its sex in order to choose the correct bin to put it in. The problem is that the task is famously difficult: male and female chicks look exactly alike.

Well, almost exactly. The Japanese invented a method of sexing chicks known as vent sexing, by which experts could rapidly ascertain the sex of one-day-old hatchlings. Beginning in the 1930s, poultry breeders from around the world traveled to the Zen-Nippon Chick Sexing School in Japan to learn the technique.

The mystery was that no one could explain exactly how it was done. It was somehow based on very subtle visual cues, but the professional sexers could not say what those cues were. They would look at the chick’s rear (where the vent is) and simply seem to know the correct bin to throw it in.

And this is how the professionals taught the student sexers. The master would stand over the apprentice and watch. The student would pick up a chick, examine its rear, and toss it into one bin or the other. The master would give feedback: yes or no. After weeks on end of this activity, the student’s brain was trained to a masterful—albeit unconscious—level.

Meanwhile, a similar story was unfolding oceans away. During World War II, under constant threat of bombings, the British had a great need to distinguish incoming aircraft quickly and accurately. Which aircraft were British planes coming home and which were German planes coming to bomb? Several airplane enthusiasts had proved to be excellent “spotters,” so the military eagerly employed their services. These spotters were so valuable that the government quickly tried to enlist more spotters—but they turned out to be rare and difficult to find. The government therefore tasked the spotters with training others.

It was a grim attempt. The spotters tried to explain their strategies but failed. No one got it, not even the spotters themselves. Like the chicken sexers, the spotters had little idea how they did what they did—they simply saw the right answer.

With a little ingenuity, the British finally figured out how to successfully train new spotters: by trial-and-error feedback. A novice would hazard a guess and an expert would say yes or no. Eventually the novices became, like their mentors, vessels of the mysterious, ineffable expertise.

The Knowledge Gap
There can be a large gap between knowledge and awareness. When we examine skills that are not amenable to introspection, the first surprise is that implicit memory is completely separable from explicit memory: You can damage one without hurting the other.

Consider patients with anterograde amnesia, who cannot consciously recall new experiences in their lives. If you spend an afternoon trying to teach them the video game Tetris, they will tell you the next day that they have no recollection of the experience, that they have never seen this game before—and, most likely, that they have no idea who you are, either. But if you look at their performance on the game the next day, you’ll find that they have improved exactly as much as nonamnesiacs. Implicitly their brains have learned the game: The knowledge is simply not accessible to their consciousness. (Interestingly, if you wake up an amnesic patient during the night after he has played Tetris, he’ll report that he was dreaming of colorful falling blocks but will have no idea why.)

Of course, it’s not just sexers and spotters and amnesiacs who enjoy unconscious learning. Essentially everything about your interaction with the world rests on this process. You may have a difficult time putting into words the characteristics of your father’s walk, or the shape of his nose, or the way he laughs—but when you see someone who walks, looks, or laughs the way he does, you know it immediately.

Flexible Intelligence
One of the most impressive features of brains—and especially human brains—is the flexibility to learn almost any kind of task that comes their way. Give an apprentice the desire to impress his master in a chicken-sexing task and his brain devotes its massive resources to distinguishing males from females. Give an unemployed aviation enthusiast a chance to be a national hero and his brain learns to distinguish enemy aircraft from local flyboys. This flexibility of learning accounts for a large part of what we consider human intelligence. While many animals are properly called intelligent, humans distinguish themselves in that they are so flexibly intelligent, fashioning their neural circuits to match the task at hand. It is for this reason that we can colonize every region on the planet, learn the local language we’re born into, and master skills as diverse as playing the violin, high-jumping, and operating space shuttle cockpits.

The Liar in Your Head
On December 31, 1974, Supreme Court Justice William O. Douglas was debilitated by a stroke that paralyzed his left side and confined him to a wheelchair. But Justice Douglas demanded to be checked out of the hospital on the grounds that he was fine. He declared that reports of his paralysis were “a myth.” When reporters expressed skepticism, he invited them to join him for a hike, a move interpreted as absurd. He even claimed to be kicking football field goals with his paralyzed leg. As a result of this apparently delusional behavior, Douglas was dismissed from his seat on the Supreme Court.

What Douglas experienced is called anosognosia. This term describes a total lack of awareness about an impairment. It’s not that Justice Douglas was lying—his brain actually believed that he could move just fine. But shouldn’t the contradicting evidence alert those with anosognosia to a problem? It turns out that alerting the system to contradictions relies on particular brain regions, especially one called the anterior cingulate cortex. Because of these conflict-monitoring regions, incompatible ideas will result in one side or another’s winning: The brain either constructs a story that makes them compatible or ignores one side of the debate. In special circumstances of brain damage, this arbitration system can be damaged, and then conflict can cause no trouble to the conscious mind.

Now Batting: Your Subconscious
On August 20, 1974, in a game between the California Angels and the Detroit Tigers, The Guinness Book of Records clocked Nolan Ryan’s fastball at 100.9 miles per hour. If you work the numbers, you’ll see that Ryan’s pitch departs the mound and crosses home plate—60 feet, 6 inches away—in four-tenths of a second. This gives just enough time for light signals from the baseball to hit the batter’s eye, work through the circuitry of the retina, activate successions of cells along the loopy superhighways of the visual system at the back of the head, cross vast territories to the motor areas, and modify the contraction of the muscles swinging the bat. Amazingly, this entire sequence is possible in less than four-tenths of a second; otherwise no one would ever hit a fastball. But even more surprising is that conscious awareness takes longer than that: about half a second. So the ball travels too rapidly for batters to be consciously aware of it.

One does not need to be consciously aware to perform sophisticated motor acts. You can notice this when you begin to duck from a snapping tree branch before you are aware that it’s coming toward you, or when you’re already jumping up when you first become aware of a phone’s ring.

Reprinted by arrangement with Pantheon Books, a division of Random House, Inc., from Incognito by David Eagleman. Copyright © 2011 by David Eagleman.