The impressive connections between mental processes such as perception, learning, memory, and
reading and specific brain processes are evidence for the identification of mind and brain only if
brain changes cause mental changes. But perhaps there is only correlation here rather than causation.
The fact that ice cream consumption and drowning frequency are correlated does not show that one
causes the other, as they have a common cause in high temperatures. A dualist could argue that all the
empirical studies described above merely show that brain processes correlate with mental ones
without brain's being the exclusive cause of mind. In scientific reasoning, the best way to show
causation rather than mere correlation is to introduce an intervention, showing that manipulating one
factor leads to a change in another factor. In psychology and neuroscience, there are both technical
and ethical reasons why it is often hard to show that manipulating the brain can produce mental
changes.
However, people frequently engage in such manipulations when they take drugs that have
psychological effects. If you use recreational drugs such as alcohol or therapeutic drugs such as
antidepressants, you are producing a physical change in your brain that changes your mental state in
predictable ways. A quick review of how drugs affect the brain and thereby change mental states
provides evidence that the connection between brain and mind is causal and not just correlational.
Much is now known about the neural and molecular mechanisms that draw people to recreational
drugs. When you have a glass of beer, wine, or whiskey, the alcohol quickly affects your brain
chemistry. Because of increased concentrations of the neurotransmitter dopamine, there is increased
activity in the nucleus accumbens, a brain area associated with feelings of pleasure. Alcohol also
increases activity of the neurotransmitter GABA, which enables some neurons to inhibit the firing of
other neurons. You then get greater inhibition of neural firing, which in small doses of alcohol can
produce relaxation but in large doses can lead to lack of coordination, slurring of words, and even
passing out. Other neurotransmitters that are altered by alcohol include serotonin and norepinephrine.
Extensive studies with animals and humans support the following causal chain: drinking alcohol
changes your brain processes and thereby changes your thinking. Similarly, we know that people
become addicted to smoking cigarettes because nicotine stimulates acetylcholine receptors and
increases dopamine levels, producing a physical dependency.
The neuropsychological mechanisms triggered by illegal drugs are also well understood.
Stimulants such as cocaine and amphetamines, including the popular drug Ecstasy, increase brain
concentrations of the pleasure-inducing neurotransmitter dopamine, as well as other energizing
neurotransmitters such as norepinephrine. Dependency on such drugs can arise because depletion of
these neurotransmitters produces cravings for more of the drug. Opiates such as heroin stimulate
special receptors in the brain leading to release of dopamine and subsequent feelings of pleasure and
relaxation, producing strong inclinations toward addiction. Enough is known about how such
powerfully addictive drugs work that we can confidently say that people use them to manipulate
mental states by changing their brain states.
In psychiatry, drugs used to treat mental illnesses also perform such manipulations. For depression,
millions of people take drugs like Prozac and Zoloft that inhibit the reuptake of the neurotransmitter
serotonin. How these drugs alleviate depression may involve the production of new neurons in the
hippocampus as well as increased availability of serotonin in the synaptic gaps between existing
neurons. Other antidepressants, such as MAO inhibitors, affect neurotransmitters in different ways.
Bipolar disorder, formerly known as manic depression, can be effectively treated with lithium, which
affects various neurotransmitters. The often devastating disease schizophrenia can sometimes be
treated with drugs like Thorazine and Risperdal that inhibit dopamine and also can affect levels of
other neurotransmitters. Increasing concentrations of dopamine can alleviate the symptoms of
Parkinson's disease.
Thus the use of recreational and therapeutic drugs provides overwhelming evidence that changing
brain processes causes changes in mental processes. Of course, the precise effects of drugs often
depend on expectations, as when people get more drunk than normal on a small amount of alcohol just
because of their social surroundings. So it is legitimate to say that mental processes cause brain
processes too. After all, the mind-brain identity theory just says that mental processes are brain
processes, and there is no problem in saying that brain processes cause other brain processes. More
importantly, these expectation effects provide no evidence for reintroducing the soul or other
nonmaterial substance into explanations of brain changes, because beliefs can be understood as neural
processes.
In this section I have only scratched the surface of the kinds of explanations that neuroscience is
increasingly able to give of diverse kinds of thinking. Those wanting more detail should consult
textbooks and journals in cognitive neuroscience and psychopharmacology, which will provide
pointers to thousands of experiments that investigate the neural bases of perception, memory, learning,
emotion, and other mental processes. The hypothesis that minds are brains is part of a highly
successful and rapidly expanding research program that has been generating neural explanations for a
wide range of mental phenomena. Experimental methods used by this research program include not
only brain scans that can identify correlations between thinking and neural activity, but also
transcranial magnetic stimulation that can cause changes in thinking by noninvasive alteration of the
electrical activity of neurons. In this technique, electromagnetic pulses are used to disrupt neural
firing, causing changes in cognitive processes such as vision and memory.
Later in this book I will provide more evidence supporting mind-brain identity. Chapters 4–6 will
provide fuller accounts of how brains know the world, have emotional experiences, and make
decisions. Proponents of the soul hypothesis cannot avoid the evidence that links such aspects of mind
with brain processes, but they have to say that the brain hypothesis is not by itself sufficient to explain
everything about thinking. Dualism maintains that people consist of both minds and bodies, or more
specifically souls and brains. Let us now consider some evidence that might support dualism over the
simpler identification of minds with brains