Is Psychology an Empirical Science?

The answer is “yes,” although it is different from physics and chemistry, and historically there has been considerable confusion surrounding this issue.  In 1948 Carl Hempel and Paul Oppenheim formulated what they called the “deductive nomological model” of scientific explanation (Hempel & Oppenheim, 1948).  A deductive nomological model is one in which a single event is subsumed under a general law.  It is fully accounted for by applying the general law to a set of initial premises or conditions, which then logically result in an observed phenomenon.  The premises must be empirically true, the conclusion logically derivable from the premises, and the general law must be a necessary component of the explanation.  Physics is the paradigm case of a science where laws of nature can be devised and applied deterministically.  Thus (simplifying), Newton apocryphally saw an apple starting to fall from a tree (the initial conditions); it descended to the ground, in some versions of the story, hitting him on the head (the observation); whereupon he devised the inverse square law (the general law) (Newton, 1687/1999).  The general law is necessary to explain what happened to the apple, and the future resting place of the apple (on the ground) is logically derivable from the general law as applied to the initial conditions (the apple falling off the tree).  Newtonian mechanics proved useful not only in clarifying what happens when subsequent apples altered their position in space and time, but also in devising hypotheses to predict specific instances where the same general law applies, for example, the trajectory of cannonballs or launching rockets to the moon.  These hypotheses can be tested experimentally.

This schematic is more problematic, however, when applied to other domains.  In 1949 Ernst Nagel hypothesized there was a hierarchy of intertheoretical explanation, with derivative sciences (such as “life science,” “natural science” and “social science”) “reducing” to more fundamental ones using “bridge laws” to connect them (Nagel, 1949, 1961).  Thus, for example, psychology is explainable in terms of neuroscience; neuroscience, in terms of molecular biology and biochemistry; molecular biology and biochemistry, in terms of biology; biology, in terms of chemistry; and chemistry, in terms of physics.  Since then there has been considerable debate (summarized at Schafner, 2001 and Darden & Tabery, 2009) as to whether life sciences are amenable to the deductive nomological model.  The main reason why it is thought they might not be is because they are not describable in terms of strict, universally applicable causal laws.  Kandel, Schwartz & Jessell (2000), for example, do not set forth any such principles governing the operations of the human brain.

Even more difficult is the status of cognitive science.  Whatever their parameters, neurophysiological processes have the potential to result in psychological states.  The former could not exist without the latter.  There is nowhere else for them to come from; they do not result, for example, from the process of digestion.  The precise nature of this relationship, however, creates several vexing problems.  Even as psychological states interact with neurophysiological processes, and in some sense are caused by them, there are no strict psychological laws to predict or explain them.  They are untethered or “multiply realizable,” because an initial set of neurophysiological conditions has the potential to result in different psychological outcomes, and any given psychological state may be grounded in different neurophysiological processes.  Because of this underdetermination, what might be called “psychological libertarianism” is compatible with its counterpart, “neurobiological determinism,” and vice versa.

In his paper “Mental Events” (1970) Donald Davidson characterized this as the “nomological irreducibility of the mental” or the “anomalism of the mental.”  Psychological processes must be explained, to the extent they can, on two different but interdependent levels of description: a physical one, and a mental one.  In principle, one neurophysiological process might be explainable in terms of another, and one psychological state in terms of another.  Particular psychological states also might be explainable in terms of particular neurophysiological processes.  However, conceived as a heteronomic class, the former only are indeterminately related to the latter.  As a consequence, Davidson (1974) writes: “[T]he social sciences cannot be expected to develop in ways exactly parallel to the physical sciences, nor can we expect ever to be able to explain and predict human behaviour with the kind of precision that is possible in principle for physical phenomena.” (p. 42)

Later, Joseph Levine (1983, 1999) characterized this as an “explanatory gap.”  As an epistemological thesis, it is impossible to determine whether statements that particular psychological states are derived from particular neurophysiological processes, are true.  Writes Levine: “Even if conceivability considerations do not establish that the mind is in fact distinct from the body, or that mental properties are metaphysically irreducible to physical properties, still they do demonstrate that we lack an explanation of the mental in terms of the physical.” (Levine, 1999, p. 3)  See also Block & Stalnaker (1999).

The anomalism of the mental and the explanatory gap have given rise to a number of countervailing objections and alternative theories.  Illustrative are epiphenomenalism, supervenience, emergence, eliminative materialism and externalism.  “Epiphenomenalism” is the idea that psychological states are nothing more than effects or by-products of neurophysiological processes (Honderich, 1981; McLaughlin, 1985).  “Supervenience” is the idea that while a change in a neurophysiological process will result in a change to a resulting psychological state, the converse is not true (Kim, 1989; Child, 1994).  “Emergence” is the idea that even though they are not reducible to them, psychological states nonetheless arise from neurophysiological processes (Chalmers, 2006; Kim, 2006).  Under all three concepts, psychological states are irrelevant or even illusory, because they play no role (or make no difference) in the explanation of neurophysiological processes, psychological states, or human behavior.  They are causally inert, or empty.

If this is so, then where did they go?  Eliminative materialism holds they can be replaced entirely by descriptions of neurophysiological processes (Churchland, 1981; Stitch, 1983; Churchland, 1986; Crick & Koch, 1990).  Externalism holds they depend primarily on social usage and environmental contexts (Putnam, 1975; Burge, 1979).  Another view is we might never be able to untangle the psycho-physiological link, in principle (McGuinn, 1989).  These theories in turn are subject to numerous disputes and disagreements, summarized at e.g. Feser (2005), Kim (2005) and Searle (2005), the main one of which is they ignore what we would like to think of as distinctive and ineliminable features of mind, such as the quality of subjective experience and its intentionality or directedness.

In my opinion much of this controversy results from a basic category mistake.  As Searle observes, it assumes science only can study “objective” phenomena, such as the neurophysiology underlying cognitive events.  Science is “epistemically objective” in that it uses replicable methods and procedures to obtain and verify results.  Brain anatomy and brain chemistry in turn are “ontologically objective” in that they are about discoverable facts, which exist independently of any particular person’s mental or psychological state.  Mental states, however, are “epistemically subjective” because the person who is having them is the only one who can experience them.  They also are “ontologically subjective” because they are real only for that person; my feelings aren’t yours, and vice versa.  They might be revealed, for example, by a person’s behavior, or described by a person’s self-report.  They cannot, however, be independently accessed or observed.  This being so, there is no reason to conclude there cannot be an epistemically objective science about mental phenomena that are ontologically subjective – which is what psychology is all about.

References

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