The Brain Revolution
Your brain is a mass of cells inside your skull and weighs around 1.4 kilograms, or 3 pounds.
Common sense insists that your mind, with all its amazing powers of thinking and feeling, cannot just
be your brain. The contrary belief that minds are souls is firmly held by the large majority of people
who belong to theistic religions, and by many philosophers since Plato and Descartes. They allow
that the mind may be closely associated with the body and especially with the brain, but insist that
mind and brain are not the same because they have different properties. Your brain has mass, consists
of matter and energy, and ceases to function when you die; whereas your soul weighs nothing, is not
subject to physical laws, and survives your death. Most people today are dualists, believing that a
person consists of both a spiritual mind and a physical body.
In contrast, most psychologists and neuroscientists are materialists and believe that minds are
brains: the mind is what the brain does. General acceptance of this view would amount to the most
radical conceptual revolution in the history of human thinking. Previously, the two most sweeping
scientific revolutions were Copernicus's rejection of Ptolemy's view that the earth is the center of the
universe, and Darwin's rejection of the religious view that humans were specially created by God.
According to modern astronomy, the earth is just another planet circling the sun, which is just one of
billions of stars in billions of galaxies. According to Darwin, humans are just another biological
species evolved through natural selection. The Brain Revolution now in progress is even more
threatening to humans' natural desire to think of ourselves as special, for it implies that our treasured
thoughts and feelings are just another biological process. Unsurprisingly, even some nonreligious
thinkers find disturbing the view that minds are brains, despite mounting evidence for such
identification. Not only immortality but also highly compelling doctrines of free will and moral
responsibility have been tied to the idea of minds as souls. The lure of dualism is powerful.
This chapter will argue that the hypothesis that minds are brains has far more explanatory power
than does its main competing hypothesis that minds are souls. Later I will also consider two
prominent materialist views that resist identifying minds with brains: the functionalist view that minds
can be processes in many different physical systems, and the embodiment view that minds are states
of the whole body. I think that neither of these views contradicts my main claim that human minds are
brains, which is, however, radically incompatible with the commonsense view that minds are not
physical objects.
Philosophers call the claim that states and processes of the mind are identical to states and
processes of the brain the identity theory. Mind-brain identification follows a long line of theoretical
identifications that have marked scientific progress: sounds are waves; combustion is chemical
combination with oxygen; water is H2O; heat is motion of molecules; lightning is electrical discharge;
light is electromagnetic energy; influenza is a viral infection; and so on. Each of these identities is
part of a larger theory that was accepted because it provided a better explanation of the relevant
evidence than did competing theories. Similarly, I will argue that the claim that minds are brains is
part of a rich theory that provides explanations for many mental phenomena, including perception,
memory, learning, inference, and emotion. Once this identification is established, we can consider the
radical implications for traditional philosophical questions about reality, knowledge, morality, and
personal meaning. Eventually, we will be able to learn from the neural processes that underlie love,
work, and play why they are such important realms of human life.
Evidence That Minds Are Brains
We believe that water is H2O and that lightning is an atmospheric electrical discharge because these
identifications are parts of accepted theories with substantial explanatory power. The connection
between lightning and electricity was first noticed in 1746 when the Dutch physicist Peter van
Musschenbroek developed the Leyden jar as a way of storing static electricity. Benjamin Franklin's
famous kite experiment in 1752 provided the first direct evidence that lightning is a discharge of
electricity. This hypothesis explained not only why sparks flew from the key that Franklin had
attached to the kite he flew in a thunderstorm, but also a broad array of observations about lightning,
such as its bright flashes, its production of loud sounds, and its ability to injure people. Demonstrating
that minds are brains is more complicated but is based on the same kind of reasoning, providing a
large array of evidence for which this hypothesis is part of the best explanation.
How Brains Explain
It was not always obvious that brains have much to do with thinking. Aristotle believed that the
primary organ supporting thought was the heart rather than the brain, whose main function was to cool
the blood. By the sixteenth century, however, rough connections between the brain and thinking were
generally recognized—for example, that vision and hearing depend on brain anatomy. Understanding
of how brains work began only at the end of the nineteenth century when the development of new
chemical techniques for staining cells made possible identification of the cells that constitute brains.
It was decades before the electrical nature of brain cells—neurons—was appreciated. Only with the
development of computers did it become possible to formulate and test detailed hypotheses about
how the interactions among large numbers of neurons might be able to support different kinds of
thinking.
When the psychologist Stephen Pinker was on the TV show The Colbert Report in 2007, Stephen
Colbert insisted that he explain, in five words or fewer, how the brain works. Pinker's brilliantly
concise response was “Brain cells fire in patterns.” Neurons are different from the cells that make up
other bodily organs in that they build up electrical charges; they can pass these on to other neurons
that are connected to them. Firing is a kind of electrical discharge. The flow of electrical charge in
neurons is only a few millivolts, compared to as much as a billion in lightning flashes, and also
differs in being directed along pathways formed by the thousand or so connections that a neuron has
with other neurons. These connections are called synapses. Figure 3.1 depicts how one neuron can
send messages in the form of electric signals to another neuron by means of synaptic connections.
3.1 A functional model of two connected neurons.
A firing neuron does not simply send a spark across to another neuron but rather sends a chemical
signal in the form of neurotransmitters that flow from the firing neuron across the synaptic gap to the
neurons to which it is connected. Using these signals, one neuron can either excite the neurons to
which it is connected, increasing their electrical activity, or inhibit the activity of the connected
neurons. Whereas a lightning flash is like a single trumpeter producing a loud sound with no intended
direction, the synaptic connections between neurons enable them to perform like a trained orchestra
with many coordinated musicians. Just as a band performance is a complex pattern of activity in a
group of musicians, a brain function is accomplished by patterns of coordinated firing activity in
interconnected neurons. The brain is not like a symphony orchestra that has a conductor to keep
everyone synchronized, but more like a bunch of jamming jazz musicians whose coordinated playing
emerges from their dynamic interactions.
At first it seems incredible that patterns of electrochemical activity in a bunch of cells could
generate thought. Then again, it is also not obvious that a hundred musicians playing together could
produce a beautiful symphony, or that billions of tiny water molecules in a cloud could accumulate a
huge electrical charge that generates bright flashes of lightning and loud rolls of thunder. But much is
coming to be known about how patterns of neural firing can produce complex kinds of perception,
memory, learning, inference, language, and other mental functions. In what follows I will be
extremely introductory. I don't need to convince neuroscientists or cognitive psychologists that minds
are brains, so the explanations that follow are aimed at people new to the idea that thinking might be
explained neurologically.
Perception
First consider our senses of sight, hearing, smell, taste, and touch, which are major sources of
information about the world. Much is known about the physical basis of how these senses work,
because they can be studied in nonhuman animals whose senses seem to operate much as our own do.
Here is what happens when you see a tree. Light reflects off the tree and into your eyes, where
photons stimulate some of the millions of nerve cells in the retina at the back of your eyeball. These
cells then send signals along your optic nerve to the back of your brain to the occipital lobe, which
begins a complex process of interpreting the retinal input using a series of regions that include parts
of the temporal lobe (see figure 3.2). Eventually, the result is a pattern of activation of neurons in the
several regions that reactivates an approximation to the pattern of neural firing that constitutes your
concept of a tree, allowing you to identify the observed object as a tree.
3.2 Rough sketch of some major regions of the brain. For a more detailed diagram, see figure 5.1.
Smell similarly involves the stimulation of receptor cells and subsequent processing to enable
recognition and storage. When you sniff a banana, for example, molecules of it are drawn into your
nose where they stimulate cells called olfacto