Step 1 in the study of consciousness is to identify what it is. This can be done by recognizing consciousness’s defining features.

Consciousness has two defining features: (1) It is subjective, and (2) it involves more than the mere detection of stimuli in the environment.

SUBJECTIVITY. Consciousness is personal. It exists, and must be assessed, from the perspective of the individual person—the “subject”—who is having the conscious experience. This is what it means to say that consciousness is “subjective” (Mandik, 2001). Only you can identify the contents of your conscious experience and what it is like to experience them.

In this respect, consciousness is relatively unique. Many other personal qualities are objective; that is, someone observing you can provide an independent, unbiased view of those qualities. This is not true for consciousness, however. An example illustrates the difference between subjective experience and objective facts.

Sometimes, as a result of injury or surgery, people lose a limb. More than 90% of these people experience phantom limb syndrome, the subjective conscious experience that the amputated limb is still attached to their body (Ramachandran & Hirstein, 1998). They often experience pain in the limb—despite its having been amputated. (The pain is produced within the brain, not the limb itself; see Chapter 3.)

The amputation is an objective fact. Someone other than the patient can assess whether the limb is still attached. If a patient claims, “Doctor, my limb is actually still attached,” the doctor can check the true state of affairs and say, “Sorry, you’re wrong; it turns out that it is not attached.” The pain, however, is subjective. If the patient says, “Doctor, I’m in pain,” there is nothing for the doctor to “check” to verify the statement, and it would make no sense for the doctor to respond, “Sorry, you’re wrong; it turns out that you are not in pain.” The patient is the one-and-only expert on his or her own conscious experience of pain.

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This is what it means for pain, and other conscious experiences, to be subjective. Such experiences occur, and can be verified, only from the point of view of the individual conscious subject.

MORE THAN MERE “DETECTION” OF THE ENVIRONMENT. The second feature of consciousness involves a distinction between (1) detecting and responding to events, and (2) feeling something after the event is detected.

Many organisms and human-made objects detect stimuli and respond to them. An “electric eye” detects you when you walk through a doorway into a store and responds by ringing a bell, alerting the storekeeper to your presence. A thermometer detects frigid temperature and responds by displaying “5°.” A Venus Flytrap detects an insect on its surface and responds by closing its leaves, trapping the insect.

These objects detect and respond to events yet are not conscious. To see this, compare your experience on a cold day to that of the thermometer. Like the thermometer, you detect frigid temperature and say, “It’s, like, 5 degrees out here!” But you do something the thermometer doesn’t; you experience the cold. You feel what it “is like” to be out in the frigid temperature. That’s not true for the thermometer. Except as a joke, you wouldn’t say, “I wonder what it’s like for my poor thermometer to be outside on such a cold day.”

This simple example makes an important point. Consciousness involves more than just detecting and responding to events. It involves feelings. The scientific study of consciousness is the study of what it feels like to be a conscious being (Chalmers, 2010).

Now that we have learned what consciousness is, we can devote the remainder of this chapter to the main topics in the study of consciousness: the question of which beings have conscious experience; psychological and biological processes in consciousness; the dreamy alteration in conscious experience that is sleep; and alterations in conscious experience caused by meditation, hypnosis, and drugs.

Earth is home to billions of creatures. It also houses billions of man-made devices—computers, smartphones, robots—that process information. Which of these are conscious?

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PEOPLE. The one being that you can be sure is conscious is yourself; you know that you experience the world. Your own consciousness is a basic fact of life.

The French philosopher René Descartes made this observation about 400 years ago. He was searching for a secure basis of knowledge—facts that he knew, for certain, were true. Descartes understood that some things that appear true could be false. For instance, the earth appears flat. The sun appears to turn red when it sets. But these are illusions, not facts. Was there anything he could know was true, for sure?

Descartes judged that the only fact of which he could be certain was the fact of his own existence and conscious experience. Even if he tried to doubt it—asking himself, “Hmm, do I, Descartes, even exist?”—the act of doubting, Descartes concluded, proved that he really did exist, because somebody had to be doing the doubting, and that person had to be Descartes himself. “I think,” Descartes claimed, “therefore I am.”

You can engage in the line of thinking Descartes pioneered and easily conclude that your own consciousness exists, too. You’re likely not alone. One can entertain the thought that “I alone exist; everybody else is a figment of my imagination.” But we will ignore that unlikely possibility and presume that all biologically normal humans possess consciousness. When asking, “Who has consciousness?” you confidently can answer, “people.”

Human consciousness is pervasive; it occurs almost all the time. When awake, you’re generally conscious of your surroundings, your feelings, and the thoughts running through your head. When asleep, you spend much of the time dreaming, which is a form of conscious experience; although not awake, you are experiencing the vivid, sometimes weird images and storylines of your dreams.

There is another sense in which consciousness is pervasive: It starts early in life and persists throughout the life span. Some evidence suggests that humans have conscious awareness before birth. Third-trimester fetuses, for example, are aware of sounds near the mother’s abdomen (Eliot, 1999). They also appear to be aware of musical sounds. In a study, researchers used an ultrasound machine to observe the reactions of 27- and 35-week-old fetuses to the musical notes middle C and high C (Shahidullah & Hepper, 1994). All fetuses reacted by moving their arms and legs when middle C was played. When the researchers repeated middle C a number of times, all infants habituated to the sound, that is, they responded to it less and less. (Habituation occurs at all ages; see Chapter 7.) When the experimenters switched notes to high C, older fetuses, but not the 27-week-olds, responded once again by moving their arms and legs. This suggests that the older fetuses were consciously aware of not only the sound, but also the difference in sound; they were conscious of the fact that middle C and high C differed.

Studies of infants complement this research on fetuses. When adults become consciously aware of a stimulus, they display a distinctive pattern of brain activity. A similar pattern of brain activity in response to stimuli has been observed in infants as young as 5 months old (Kouider et al., 2013). This finding further suggests that conscious awareness is evident early in life.

This capacity for consciousness usually lasts a lifetime. However, people sometimes lose it. Injury or illness can extinguish conscious experience, as you saw in this chapter’s opening story. Researchers did identify a person who had been diagnosed as being in a persistently vegetative state—that is, a state lacking conscious awareness—yet whose brain activity indicated that he actually was consciously aware of the world. Unfortunately, many other patients in this study (Owen et al., 2006) were not. They showed no distinctive brain activity in response to the researchers’ probing. Both behavioral and brain evidence indicated that these other patients, although alive, had lost the capacity for consciousness.

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NONHUMAN ANIMALS. How do we know that nonhuman animals are conscious? They appear to be conscious; your dog seems excited when you arrive home and tired after she runs around in a park. But appearances could be deceiving. A motion-sensitive electronic device could appear conscious if programmed to excitedly shout “Hello!” when you pass by. What firm evidence is there, then, that animals have a conscious mind?

There are two types of evidence. One is behavioral. When animals perform tasks, they are distractible in much the same way that people are. Researchers observe that animals sometimes lose track of important information (e.g., the location of offspring) when engaged in an activity unrelated to that information (e.g., obtaining food for oneself; Baars, 2005). The parallel between human and animal distractibility suggests that both human and animal mental life consists of conscious experience of limited size. We and our furry friends can focus on only a small amount of information at any given time.

The second source of evidence is biological. The brain systems needed for human consciousness (see below) exist, in quite similar form, in mammals other than humans. This cross-species similarity suggests that all mammals have conscious feelings (Baars, 2005).

When discussing animal consciousness, it is important to distinguish consciousness from self-consciousness. Consciousness, as you’ve learned, occurs when an organism has subjective experiences, or feelings. Self-consciousness is different; it refers to thinking about yourself. People are thinking self-consciously when they contemplate themselves, their experiences, and how they appear to others.

You can have conscious feelings without self-conscious thoughts. For instance, if you are injured, you immediately have a conscious feeling of pain. Afterward, you might have self-conscious thoughts such as “I’m hurt!” or “What happened to me?” The initial pain, before the thoughts, shows that conscious feelings can occur without self-conscious thinking.

Few animals show any sign of self-conscious thought. In fact, few seem even to recognize themselves. In a mirror self-recognition test, researchers first expose an animal to its image in a mirror. They then make an unusual mark on the animal and place it in front of the mirror again, to determine whether it recognizes that the mark is a mark on itself. (Rubbing the mark on its body, for example, would be a sign of recognition.) Only a few species other than humans—gorillas, chimpanzees, orangutans, and probably dolphins (Reiss & Marino, 2001; Suddendorf & Butler, 2013)—can pass the mirror recognition test. Household pets such as dogs and cats cannot.

Even nonhuman species that pass the mirror self-recognition test lack self-conscious thinking of the human variety. A central feature of human self-conscious thought is mental time travel, the ability to project yourself backward or forward in time in your mind (Suddendorf & Corballis, 2007). People commonly think about themselves in the past and the prospective future, dwelling on past events, how they could have acted differently, and how they will cope with events next time (Kahneman & Miller, 1986). There is no convincing evidence that animals do any of this. Humans appear to be unique in their ability to engage in mental time travel.

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In sum, then, humans have a unique form of self-conscious mental life. But we are not unique in having conscious experiences. A vast number of animal species consciously experience pains and pleasures, desires and fears.

Not all animals have consciousness, however. Many simple organisms, such as insects or worms, lack the brain systems required for conscious experience. They detect and react to events in the environment, but scientific findings provide no firm grounds for concluding that they have conscious feelings. Some argue that even fish, a relatively complex class of organisms, lack the brain structures required for conscious feelings. Analyses of the nervous system of fish suggest that they merely react to the environment reflexively, experiencing neither pleasure nor pain (Rose, 2002).

ROBOTS? If you walk into a Honda office in Japan, ASIMO, the receptionist, will stroll up, say hello, shake your hand, and show you around the place. ASIMO will answer simple questions and, with some practice, may even learn to recognize you the next time you stop by.

This wouldn’t be all that impressive for a flesh-and-blood receptionist. But—as you may have guessed from his wacky name—ASIMO is a robot.

Is ASIMO conscious? It certainly looks like it. If something walks up to you, shakes your hand, and answers your questions, it would be reasonable to guess that it is conscious. Yet ASIMO surely lacks consciousness. So do his electronic relatives: computers, smartphones, chess-playing software programs, robotic vacuum cleaners, and the like. These devices do process information. They encode incoming information into memory, combine it with stored information, and execute programs that manipulate the information. But they do not have feelings; they are no more conscious than a thermometer.

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Information processing, by itself, is not sufficient to produce conscious experience (Chalmers, 1996; McGinn, 1999; Searle, 1980). Information processing is merely the manipulation of symbols (numbers or words) according to logical rules. Consciousness is different; it involves not only symbol manipulation, but also emotions, sensations, and other feelings. In fact, logical reasoning is not even necessary for conscious experience. An animal bitten by a predator experiences pain immediately, prior to engaging in any steps of information processing to determine, for example, who the attacker was.

Present-day robots (and other information-processing machines) thus do not have conscious feelings. What about robots of the future? Many believe they will never be conscious. Others are optimistic, suggesting that with extra circuits, today’s robots can be made to experience feelings (Parisi & Petrosino, 2010). Stay tuned!