Consilience

"Consilience of inductions" is a phrase that was invented by the nineteenth-century English historian and philosopher of science William Whewell (1794–1866; pronounced "Hule"), and introduced in his Philosophy of the Inductive Sciences (1840). Whewell was trying to capture the notion of what Isaac Newton (1642–1727) had labeled a "true cause," or vera causa, the kind of cause that supposedly lies at the center of the very best kinds of scientific theories. Whewell's friend, the astronomer and philosopher John F. W. Herschel (1792–1871), had argued (in an empiricist fashion) that a true cause is something that we ourselves experience directly or analogically. We know for instance that a force is pulling the moon toward the earth, because we have had direct experience of the tug of a string as we are whirling around with a stone tied to its end.

Whewell, to the contrary, in a rationalist fashion, wanted to characterize a true cause as something that is not necessarily experienced at all, but that is adequate to explain the empirical facts as we know them. He argued therefore that such a cause must be at the center of a scientific explanation, explaining all of the disparate facts and in turn being explained by them. If such a cause can predict and explain new and excitingly unexpected facts, then so much the better. Such a fan-like construction, with the cause at the apex, Whewell labeled a consilience of inductions. He argued also that such a situation is as simple as it is possible for a theory to be, and hence a true cause is also marked by the fact that it is the most economical of explanatory forces, with many parts being reduced to one fundamental mechanism.

For Whewell and Herschel, the paradigmatic example of a true cause was Newtonian mechanics, which used the one force—gravitational attraction—to explain not just the heavenly motions of the planets but also the earth-bound motions of such things as cannonballs. Johannes Kepler's (1571–1630) laws of celestial motion and Galileo Galilei's (1564–1642) laws of terrestrial motion were brought beneath one force. But Whewell and Herschel themselves were arguing over the science of their day, most particularly the nature and causes of light. The beginning of the nineteenth century was the time when the wave theory of Christiaan Huygens (1629–1695) was conquering the particle theory of Isaac Newton. The problem is that no one sees waves. Herschel argued that we have experienced analogous phenomena to light waves, for instance the waves of water in a pond. Hence it is reasonable to suppose that light waves produce such things as interference patterns. Whewell replied that there is no need of analogies—it is enough that the wave theory explains many phenomena that are impossible to explain with the particle theory. The patterns produced by something such as Thomas Young's (1773–1829) double-slit experiment are themselves enough to confirm that light travels in waves and not in particles.

Also dividing Whewell and Herschel was the true nature of geology, with Whewell arguing that it is permissible—even obligatory—to invoke unseen upheavals (catastrophes), and with Herschel agreeing with the uniformitarian geologist Charles Lyell (1797–1875) that unlimited time is enough to produce all changes, given the kinds of causes (rain, sleet, earthquakes, and so forth) that we see around us today. A young scientist who took deep note of Whewell's arguments was Charles Darwin (1809–1882), who, realizing that no one can ever see evolution actually occurring, consciously modeled his Origin of Species (1859) on Whewell's thinking. Having introduced his mechanism of natural selection, Darwin then spent the rest of the work showing how selection explains so many different areas—instinct, biogeography, paleontology, embryology, morphology, classification, and more—and how these areas in turn confirm natural selection as a true cause. When challenged, Darwin always referred to this strategy, even though, somewhat paradoxically, Whewell himself never became an evolutionist and supposedly would not allow the book on the shelves of the Cambridge college of which he was principal.