Behaviorism dominated psychology in the first half of the twentieth century. It may seem intuitive that psychology ought to be about conscious mental life, but behav- iorists, such as John Watson (1878-1958), thought that the concern for consciousness was misguided and that those who studied consciousness (or worse—the subconscious) were not real scientists. “Consciousness,” Watson argued, is another word for “soul” (Watson and McDougall 1928, p. 15), and he thought there had been too much medieval speculation in introspectionist psychology and the philosophy of mind. Watson urged researchers to leave the soul, consciousness—or whatever you want to call this medieval remnant—to priests, shamans, or other religious people, and get on with a science of behavior instead. A real science of psychology would make third-person observations as is done in physics and chemistry—observations verifiable by independent observers.
But what about all of that which we normally think of as being part of conscious mental life—beliefs, desires, thoughts, feelings, and the like? They can be understood as dynamically evolving behavioral patterns explicable in terms of stimuli-response mechanisms. Seeing no need to keep mental concepts in behaviorist science, Watson comments:
He “ignores” them in the same sense that chemistry ignores alchemy, astronomy horoscopy, and psychology telepathy and psychic manifestations. The behaviourist does not concern himself with them because as the stream of his science broadens and deepens such older concepts are sucked under, never to reappear. (Watson 1920, p. 94)
How can this be? The commonsense picture is that mental life can be expressed— become manifest through behavior—not that mental life simply is, in and of itself, behavior. When a child weeps, we see the behavior as an expression of internally experienced pain. But Watson, in his more radical mood, holds that there is nothing more to the pain than crying behavior. To infer there is something going on inside— some incorporeal thing, in the child’s conscious mind—is superstition.
What is the role of the brain if it is not the seat of consciousness and mental life? Watson thinks we can ignore the brain. How the brain works in detail is irrelevant to scientific psychology. Although the human brain has complex wiring, we know what it is: a stimuli-response machine, mediating between input stimuli and output behavior—a neural switchboard, which receives sensory input and relays it to motor output.
In Watson’s view, you can forget about the brain if you are a psychologist. Remember that humans are animals, and ask yourself what all animal psychology shares. How could you find universal laws of psychology holding true for all animals? Perhaps you will see that the only way to achieve truly universal laws in psychology is by scrutinizing behavior. Universal laws in psychology would hold for animals other than human beings, but animals such as cats and dogs have no place in an introspectionist or psychoanalytic program, since they are incapable of conveying verbal reports. Does this mean we must deny them psychology? Watson thought not.
You may think there is something special about human language. Like Descartes, you may think that language is the outward sign of a thinking mind that exists independently of behavior, and that having such a mind is unique to humans—but Descartes was mistaken. Watson sees language ability as a behavioral skill and, although complex, it’s just bodily behavior. After all, how do you know what people are thinking and feeling? From behavior! All ascriptions of mental life to others are based on behavioral inferences, and all that you project to others about your supposed “mental processes” is behavior.
You may believe that to simply think is to engage in behaviorally empty mental activities. If so, then Watsonian behaviorism must be false. Watson did not accept this line of reasoning. He claimed that, as you think, there are micromovements in your larynx and they constitute thought: “according to my view, thought processes are really motor habits in the larynx” (Watson 1913). However, at 2 p.m. on January 10, 1946, Watson’s view was refuted when a radical experiment was conducted on a human subject at the University of Utah. The subject was administered incremental doses of d-tubocurarine (curare), rendering his skeletal muscles—including the larynx—paralyzed. Could he still think? He reported afterward that he could and had remained fully aware, albeit paralyzed: “clear as a bell.” This is a brief excerpt from the clinical log:
- 2:42 Ability to signal by slight movement of inner aspect left eyebrow almost gone. Indicates he desires the final 100 units, that he is perfectly conscious and that his sensorium is unimpaired.
- 2:44 Additional 100 units d-tubocurarine chloride given rapidly; total, 500 units.
- 2:45 Subject now unable to signal response to inquiries, due to complete skeletal muscular paralysis. Endotracheal catheter inserted with ease due to very relaxed pharynx and vocal cords, and artificial respiration continued through it. B.P., 130/84; pulse rate, 120.
- 2:48 Eyelids manually opened. Alpha rhythm of electro-encephalogram inhibited by pattern vision (object held in line of gaze). Subject stated upon recovery that he was “clear as a bell” all this period. (Scott et al. 1947, pp. 4-7)
Not surprisingly, the radical thesis of behaviorism did not hold. Behaviorism also came under attack from within itself. Edward C. Tolman (1886-1959), a behaviorist who ran rats through mazes in wheelbarrows at University of California, Berkeley, as part of learning experiments, proved the stimuli-response model wrong. Tolman’s rats developed cognitive maps—mental representations of their environment (Tolman 1948). Moreover, that they did this passively (as belted wheelbarrow riders) was solid evidence against the behaviorist reward-punishment model of learning. Tolman demonstrated that brains are not simply stimuli-response machines. We and other animals are bombarded with stimuli, but what becomes salient depends on active capacities for representing the world and acting autonomously. As a cognitive behaviorist, Tolman took pioneering steps in the direction of cognitive psychology.
A further attack on behaviorism came from the linguist and cognitive scientist Noam Chomsky (1928-), who criticized the behaviorist model of language learning by appealing to how humans seem to have innate capacities for language. How, thought Chomsky, could it be that children learn to speak without being taught grammar? According to Chomsky, each child is endowed with an innate language acquisition device1 to automatically learn whatever language the child is exposed to. The language acquisition device works for any natural language. Chomsky noted how we can understand—and utter—a great many sentences that we have never encountered. He maintained that no finite period of stimuli-response training could account for this ability.
By the end of the 1950s, the glory days of behaviorism were over. Chomsky stated that defining psychology as the study of behavior was like defining physics as the science of meter reading. Behaviorism amounted to little more than observing expressions of psychological processes. You had to understand the psychological processes themselves. This meant redefining psychology in terms of cognition. 
-  See section: “6. Further Remarks on Descriptive and Explanatory Theories” (Chomsky 1965).