Of planets and cognitions: the use of deductive inference in the natural sciences and psychology

Inferences of unobserved cognitive events in psychology are not the same as the inference of the recently discovered, but unobserved extrasolar planets in astronomy. Unlike astronomy, or the other natural sciences, psychology has no experimental foundation of laws that justify such inferences.

Magazines such as the SKEPTICAL INQUIRER encourage skepticism of beliefs and claims, ranging from the extraordinary to the quotidian, that often characterize the so-called pseudosciences. Unscientific claims also occur frequently within established disciplines such as the social sciences, in which it is fairly easy to engage in biased judgment and illogical thinking. To understand the ease with which belief can transcend science, one need only examine such false claims as facilitated communication and repressed memory syndrome, not to mention more entrenched beliefs such as psychoanalytic personality theory. Even the natural sciences are not immune to this problem, because humans are fallible and their judgments can often be clouded by personal beliefs, even in the face of contrary scientific evidence.

What distinguishes the natural from the social sciences, however, is that in the natural sciences there is a self-corrective mechanism inherent in the scientific methodology.

Because the social sciences lack such a mechanism, scientifically questionable claims and beliefs have a much longer life than those in the natural sciences.

As one who believes that psychology can and should be a natural science of behavior, I am uneasy with the fact that much of modern psychology is based upon unobservable mental events. Some psychologists may insist that inferring such events is an unavoidable pitfall of a discipline in which the area of study has historically been the mind. Others may argue that psychology simply emulates the natural sciences, where inferences are regularly made about unobservable events. A recent example in astronomy provides an opportunity to compare inferences in psychology with those in the natural sciences.

In October 1995, astronomers at the Geneva Observatory in Switzerland announced with much scientific and media fanfare the discovery of the first planet outside our own solar system. The planet is in a tight orbit around 51 Pegasi, a star in the constellation Pegasus. Just three months later, astronomers at San Francisco State University and the University of California at Berkeley announced the discovery of two other extrasolar planets orbiting the stars 70 Virginis, located in the constellation Virgo, and 47 Ursa Majoris. The astronomers speculated about the orbit and minimum mass of each planet, but could offer no other details because the planets themselves had not been directly observed.

The initial response from the scientific community was rightly one of skepticism - these were, after all, potentially momentous discoveries that required verification. Other astronomers later replicated the initial observations and measurements, although still without observing the planets directly. Although the 51 Pegasus report was strongly questioned, researchers have now inferred by similar means the presence of at least eight planets outside the solar system. Of course, it is not possible for the astronomers to have observed the planets given the available technology, because the planets' suns are too bright to allow reflected light from the planets to be seen. But if one of the axioms of scientific methodology is objective observation, then it is not unreasonable to ask: How can scientists accept these claims without direct observation?

(In late May 1998 NASA announced that the Hubble Space Telescope may have taken the first image of a planet outside our solar system, an object 450 light-years away in Taurus designated TMR-1C. That is an exciting possible discovery, but it doesn't affect my general point. Most expected extrasolar planets will remain beyond Hubble's optical reach.)

The answer is that the discoverers of these extrasolar planets applied a form of deductive inference based on scientifically proven laws and principles. Beginning with Galileo Newton, scientists have made direct observations of visible bodies and the forces they exert on one another. After conducting experiments on the motion of objects on earth, scientists were able to extrapolate the discovered laws to celestial bodies. For example, as David Palmer at Smith College points out, Isaac Newton explained the effects of the moon's gravitational pull on ocean tides using his laws of motion even though he obviously never carried out direct experiments on ocean tides. Nevertheless, we accept his interpretation as a plausible explanation based on the strength of the laws of physics he discovered under more controlled conditions.

The experimental findings of Galileo and Newton have been formalized as the laws of motion and the theory of gravity which explains them. With these laws as a scientific foundation, the discoverers of the extrasolar planets were justified in deducing, and inferring, the existence of the planets based on observations other than those of the planets themselves; namely, slight inconsistencies in the pulsations of the stars around which the planets orbit. According to David Black, director of the Lunar and Planetary Institute in Houston, scientists scrutinize a star for signs that it is affected by the gravitational tug of an orbiting planet. As the planet moves from one side of the star to the other, its gravitational pull moves the star back and forth. Astronomers are able to detect this effect as a slight wobble in the star's overall movement across the sky.