Friday, May 08, 2009

The Idea of Human Nature, Part 2

The three conflicting views of human nature--materialism, interactionism, supernaturalism--run throughout the history of natural science from Socrates to the present. In Plato's Phaedo, Socrates talks with his friends while awaiting execution. He recounts that as a young man he thought that a scientific investigation of nature would explain the cause of everything. He had hoped to explain the physical causes of all things coming into being and passing away, including the causes of animal life and the causes of human thought. He became frustrated when he found that a complete science of nature as governed by physical causes was beyond his grasp. To explain the world, Socrates insists, it is necessary to understand both physical causes and mental causes. For example, to explain why Socrates is sitting here awaiting his execution, one might describe the physical mechanisms in his body--the bones, muscles, ligaments, and so on--that control his movement. But while these physical causes are necessary in explaining why he is sitting here, they are not sufficient. It is also necessary to explain how Socrates made up his mind to accept his punishment, because this mental decision controls his physical body.

Socrates appeals to a person's ordinary experience of making up one's mind and then freely choosing to act according to that conscious mental decision. This leads people to think that the mind has a power to act that changes the physical causes of the body. Holding oneself and others morally and legally responsible for their conduct assumes that freedom of thought and choice.

This kind of Socratic thinking has led many people to conclude that human nature is characterized by a complex interaction of mind and body, mental and physical causes. The human mind acts on the human body, or the mind exerts an immaterial power that is not reducible to the material causes of the body.

Socrates was responding to a materialist or physicalist tendency that became a strong tradition in Western science. That materialist tradition gained great power during the scientific revolution of the sixteenth and seventeenth centuries. Proponents of the new science saw the universe as a mechanism that could be explained by mechanical laws working through physical causes. It seemed that much of human nature could be explained similarly without invoking an immaterial soul.

Thomas Hobbes saw nature as matter in motion governed by laws of motion such as those discovered by Galileo. Animal life, then, including human life, is "but a motion of limbs." "For what is the heart, but a spring; and the nerves, but so many strings; and the joints, but so many wheels, giving motion to the whole body." Animal motion is driven mechanically by selfish passions that goad animals to seek pleasure and avoid pain. Although human beings are moved by some of the same selfish passions, humans are unique in their capacity for reason and speech. Yet even this uniquely human intellectual capacity can be understood mechanistically as the computational manipulation of informational signals. (This is why some proponents of "artificial intelligence" today see Hobbes as the founder of the modern view of thought as computational and thus, in principle, replicable in computers and robots.)

Hobbes's materialist science of the soul seemed to be confirmed by Thomas Willis's studies of the brain. Working in England as the same time as Hobbes, Willis compared the anatomy of the human brain with that of other animal brains and combined experiments on brains with medical observations of brain-damaged patients to develop what he called "neurology." He reached five broad conclusions. First, all mental experience arises from the motion of "animal spirits" undergoing chemical changes in the brain. Second, different parts of the brain have different functions. Third, the human brain resembles other animal brains, particularly those of monkeys and apes. Fourth, this science of neurology could be used by medical doctors to cure diseases of the brain through the use of drugs that would alter the chemistry of the brain. Fifth, all this supports the general view of the "mechanical philosophy" of the seventeenth century that the human body and brain are both machines explainable by mechanical laws.

Although Willis was mistaken about many details, his broad conclusions are supported by modern neuroscience. What Willis called animal spirits can be understood as electrical and chemical signaling between neurons. Willis's observation that the brain has specialized functions has been elaborated by studies of the ways neurons are organized into modular networks with distinct functions. Willis's claim that the human brain resembles the brains of other animals can be explained by evolutionary biology. His hope that drugs could cure the diseases of the soul seems to have been fulfilled by modern psychopharmacology in its use of drugs to treat mental disorders and enhance mental function. Finally, Willis's mechanistic account of the mind has been elaborated with computer models of the mind as an information-processing system.

But while it might appear that the science of the human brain initiated by Willis proves Hobbes's materialist view of the soul, Willis was not in fact a strict materialist, because he believed that his science showed the existence of two souls. The "sensitive soul" found in all animals was purely material and therefore vulnerable to physical diseases. The "rational soul" found only in human beings was immaterial and immortal, although it depended on the sensitive soul. So Willis's account of human nature was interactionist in that he thought the material brain and the immaterial soul mutually influenced each other. He was also a supernaturalist in that he thought the immaterial soul was created by God to be immortal. (Still, it's hard to decide whether he was sincere in this, or whether he was professing supernaturalism to protect himself against scandal.)

Today, many scientists argue that natural science sustains a purely materialist view of human nature and refutes any belief in the human soul as immaterial or immortal. But some neuroscientists--for example, Wilder Penfield and John Eccles--have defended Willis's interactionist view of the mind as an immaterial cause that can act on the brain. Eccles, a Nobel-prize-winning neuroscientist, argued that modern neuroscience is compatible with belief in the self-conscious mind as an immaterial power for thinking and choosing.

What difference do these debates over the science of mind-brain interaction make for an understanding of human nature and morality? Those who argue for an immaterial soul agree with Socrates that the capacity of the mind to act outside the laws of physical nature is necessary for moral freedom. They warn against scientific materialism as a denial of free will that would make it impossible to hold people morally responsible for their conduct. They also warn that a materialistic view of human nature would promote a Hobbesian hedonism that would deprive human life of any moral dignity. And if the ultimate end of modern science is the conquest of nature, people might be tempted to use the technological power of science to alter human nature itself in ways that would be dehumanizing. This seems to confirm the fears of many people that modern science, insofar as it promotes a materialist view of human nature, subverts traditional morality. (This is the argument of people like Leon Kass.)

And yet, as I have often argued on this blog, scientific reasoning about the human mind can support traditional morality by showing how it is rooted in the human brain. Darwin showed how a natural moral sense could be implanted in human nature by evolutionary history. As naturally social animals, human beings evolved to have a natural sense of right and wrong that would support social cooperation on the basis of ties of kinship, reciprocity, and mutuality. To reinforce this cooperative behavior, they were endowed with emotional propensities to moral emotions such as love, guilt, and indignation, and they were also endowed with the intellectual capacity to formulate social norms of cooperation rooted in those moral emotions.

Some neuroscientists have found that moral experience depends on the moral emotions sustained by the emotional control centers of the brain and on the moral reasoning carried out in the prefrontal cortex of the brain. If these parts of the brain are not functioning normally, people cannot act as moral beings. For example, psychopathic criminals apparently have an abnormality in their brain circuitry that prevents them from feeling the moral emotions that support the moral conduct of normal human beings. Such scientific research suggests that morality is part of the biological nature of human beings.

A selection of some of my previous posts on the neuroscience of the mind, morality, and freedom can be found here, here, here, here, here, here, and here.

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