Lecture 19.  October 28 and 29.

Afterthoughts.   In order to make a place for psychology at the table of science, psychology’s founder, Wilhelm Wundt, proclaimed an “alliance” between philosophical psychology and physiology, fulfilling an unbroken tradition reaching back to ancient Greece.  Claimed as an ally, physiology at the same time posed a danger to psychology, threatening its autonomy as a science by reducing mental concepts to neural facts and its existence as a discipline by revealing its subject matter—the soul, or its replacement, consciousness—to be an illusion.  Redefining psychology as the science of behavior dispensed with the ghost in the machine, but only postponed psychology’s reckoning with physiology.  While the biological substrates of behavior eluded early neuroscience, they had to be there and would one day be discovered. 

The aim and concepts of reductionism were developed by philosophers in the positivist tradition that founded philosophy of science as a discipline.  The early positivists, led by August Comte, suggested that sciences could be arranged in a historical and philosophical hierarchy reflecting their relative appearances in time and their relative philosophical statuses from last developing and least basic science (sociology) to first developing and most basic science (physics).  The idea was, very roughly (details were worked out later), that the laws of group behavior (sociology) would reduce to the more basic laws governing the behavior of humans comprising social groups (psychology), whose laws in turn would reduce to the more basic laws of biochemistry governing the nervous system of each person (neuroscience), which would reduce to the laws of chemistry, and thence to the laws of the particles making up each chemical element (physics).

The problem of reduction arises when a domain is addressed by two theories, raising the question of how such theories might relate to one another.  Historical examples include theories concerning the movements of the planets, heat, and the behavior of gases.

One possibility, of course, is replacement: One theory is correct and the other is wrong, and the first, typically newer, theory replaces the other.  A paradigm instance is the replacement of the Ptolomaic, earth-centered, account of the solar system by the Copernican, sun-centered, one.  In this case the conceptual furniture of the universe was left unchanged: Moon and Mars, Jupiter and Sol remained, but their positions and motions were understood and explained in new ways.  In other instances, replacement entails the complete elimination of things posited by older theories.  For example, as atomic understanding of matter and energy progressed in the 18th century, older concepts used to explain phenomena such as heat were found to be without reference: Elimination was the fate of phlogiston and caloric and of fluidic theories of heat and electromagnetism in general.

The second possibility is reduction:  A theory might turn out to be valid at one level of description and explanation, but reducible to a more basic and more general theory.  A paradigm instance is the relation between the classical gas laws and the atomic theory of matter.  Early physicists had shown that the behavior of gases could be predicted and explained by laws using the variables pressure, temperature, and volume.  So, for example, pressure cookers maintain a gas (steam) at a constant volume so that as the air and water vapor is heated the temperature in the cooker rises; on the other hand, heating air causes a hot-air balloon’s bladder to expand.  The gas laws were mathematically precise and descriptively true.  As the atomic theory of matter developed, however, heat came to be understood as the rapidity of molecular movement in a physical body: the more rapid the motion, the higher the temperature.  Applied to gases, atomic theory explained why the gas laws were true.  In the pressure cooker, the atoms of water vapor trapped inside move faster and faster as heat is applied, and so temperature rises; in the hot-air balloon, the molecules of heated air push against the enclosing bladder, forcing it to open more, and volume increases. 

In a reduction, the reduced theory is retained in science, but is explained at a lower level of discourse (atoms rather than gasses) and is incorporated into a broader, more general, account of nature (gasses are seen to follow the same principles and are made of the same stuff as all matter without exception).  This example shows that when psychologists discuss and fear “reductionism,” they usually are discussing and fearing replacement instead.  Note, also, that replaced theories, even though known to be false, may be retained for practical use.  Calculating one’s location on the earth under Ptolomaic assumptions is much easier than under Copernican ones, and for centuries after Copernicus’ Revolution of the heavenly orbs sailors sailed the seas of a notionally earth-centered universe.

Forethoughts.  Early psychologists flirted with reductionism, but most moved away from it.  Wundt’s alliance with physiology weakened during his career.  In his early writings, he often proposed physiological accounts of mental processes such as attention, but in the end the alliance became more a matter of experimental method than of theoretical substance.  Freud was besotted with the prospect of reduction in his “Project for a scientific psychology,” but never published it, although its ghostly echoes remain in his later so-called “pure psychology.”  Behaviorists were similarly ambivalent in their relationships with physiology.  John Watson, who launced the behaviorist movement, was a materialist and sometimes talked like a reductionist and eliminativist, but it was more bluster and attitude than a real attempt to do psychology as physiology.  His student Karl Lashley, did try to carry out a reductionist program with respect to learning, but it never came to anything, probably because the research tools needed lay decades in the future.

In the later 20th century, cognitive psychologists and allied philosophers of mind declared their independence from physiology and denied that cognitive theories could be reduced to or eliminated by neuroscience.  Their most formidable argument derived from the symbol-system version of cognitive psychology, and is known as the argument from multiple realizability.  In brief, the argument is this.  In the symbol system view, cognitive processes consist in the manipulation of symbols by logical rules.  Symbol manipulation can be performed equally well by different physical devices, most notably organic brains made of tissue and electronic brains made of silicon and metal; hence, the familiar metaphor that the mind is like a computer, or, more precisely, that mind is to brain as program is to computer.  Cognitive theorizing, whether in psychology or artificial intelligence, was about formally defined symbols and rules; how symbols and rules were grounded in a brain or a computer was “mere implementation.”  At the margin, this meant that a person’s mind could, in principle, be written as a computer program and downloaded into a computer, with no resulting change in behavior. 

Important to the argument was the distinction between types and tokens.  Each person is a token of the (conceptual) type “human being;” each dime in your pocket is a token of the type “dime.”  The beauty of multiple realizability—known in philosophy as non-reductive physicalism—was that it was materialist—no soul-stuff need be invoked—yet it preserved the theoretical autonomy of psychology.  Every mental event, or token (in the sense of a piece of cognitive computation), corresponded to some physiological or electronic token, but no mental type shared across cognizers, organic or inorganic (e.g., knowing that a dime is a unit of US currency) corresponded to any physical type in the system implementing a cognitive system.  The idea is perhaps clearest in the case of computer programs.  One can play a game such as Command and ConquerÔ on a PC, an Xbox, or a Mac, and it will look and feel the same even though the underlying machine code is different in each device.  Reduction is therefore only trivially true and poses no threat to psychology.  Brains and machines carry out computations, but no theoretical gain is won by worrying about how they do so.  Description, prediction, and control, the scientific goals of theorizing, can be fully met at the cognitive level.

Nevertheless, reductionist and eliminativist proposals have been revived in the 21st century, as neuroscience has made enormous advances in understanding the physiological mechanics of mental processes. Clinical psychologists are struggling to survive in the age of Prozac.  Even economics, which would seem immune to reductionism because it deals with social entities such as money and interest rates, has within it a new approach called neuroeconomics.


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