This is a sequel to a post I wrote in June called "The explanatory reductio." The point there was this: Although an explanation is an attempt to understand an accepted fact, sometimes our inability to provide an explanation becomes a reason for rejecting the 'fact' instead. If you can't explain that beautiful creature in your bed, maybe it's not your bed. Here I offer the following observation, and provide you with some examples:
Many of the greatest intellectual insights in human history resulted from someone explaining a widely accepted fact, as a grand illusion.
1. The moon illusion
As everyone knows, the moon is larger when it appears on the horizon than when it is high in the night sky. Sometime back in human prehistory ancient people wondered: Why does the moon grow and shrink like that? One evening, while sitting at his fire after a fruitless day of hunting, the famous cave philosopher Og, had an epiphany.
Og say moon not get small. Moon only look small. Moon like prey, get small when run away.
Og was the first to articulate the appearance/reality distinction, an incredible leap in human understanding. Later astronomers accepted Og's view that the changing size of the moon is illusory, but they rejected his explanation. They believed all celestial bodies move in perfect circles, with the earth at the exact center of their orbits. Aristotle proposed a new explanation: The atmosphere acts as a lens, magnifiying the moon's appearance. Wrong again, as can be easily shown. Cover the moon with a quarter while it is on the horizon, and cover it again when it is high in the sky. The distance between the coin and your eye is the same. There is still no accepted explanation of this phenomenon, and there probably will not be one until we understand which of several different illusions relating to relative size and distance our brain circuitry is falling for.
2. Planetary retrograde
The powerful idea that heavenly bodies move in perfect spheres around the earth was dogged for centuries by the strange phenomenon of planetary retrograde. Long before Aristotle, astronomers from different cultures had observed that planets would sometimes start moving backwards. Apollonius and, later, Ptolemy explained the retrograde as a real phenomenon. Their theory of epicycles held that the planets did not move in simple circles, but in epicycles like this:
The epicycle is the smaller orbit, and at the bottom of the epicyclic orbit, the planet itself really does move backwards. Genius. And wrong. One of the great insights embedded in the heliocentric system proposed by Nicholas Copernicus is that retrograde motion is an illusion that occurs when earth's orbit either overtakes, or is overtaken by, the orbits of other planets.
3: Galilean relativity
Aristotle's physics accepted as real a variety of phenomena that turn out to be illusions. One of the most important- the basis of his categorical distinction between the heavens and the earth - is that rest and uniform motion are different states of matter. For Aristotle the natural state of an object is rest; it requires no explanation. For an object to move, and to continue moving, a cause is required. This illusion was finally shattered by Galileo (the most famous early adopter of the Copernican view) who realized that motion is not a property of an object at all; it is just an artifact of the chosen reference frame. Any object that can be described as moving in one reference frame, can be described as motionless in another. Galileo's insight was ultimately codified in Newton's First Law of Motion.
4: Darwin's theory of evolution
What accounts for the design of the universe? As most philosophy students know, William Paley argued that, just as the evident design of a watch suggests the existence of a watchmaker, so the evident design of the universe suggests the existence of a universe maker. Even David Hume, who rejected the designer hypothesis as childish anthropomorphism, admitted that the design of the universe required an explanation- and that he didn't have one. Enter Charles Darwin. In the Origin of Species Darwin provided a theory of the emergence, not of design, but of the illusion of design. The illusion of design could, he proved, result from a process involving competition, reproduction and blind, natural 'selection'. Darwin's insight effectively completed the Copernican revolution, destroying the nearly universal belief that human origins and human capacities are beyond human understanding.
5: Continental drift
What Darwin did to the eternality of species, Alfred Wegener did to the immobility of continents. Until the late 20th century every schoolkid was taught that these gigantic land masses were necessarily immobile. That uncanny fit between the west coast of Africa and the East coast of South America? Coincidence. The similar fossil distributions on adjacent coastlines? Strategically placed land bridges that have, sadly, vanished without a trace. Wegener's theory of continental drift, which hypothesized an ancient super continent called Pangaea, represented the immobility of the continents as an illusion. They simply moved too slowly for us to notice. Wegener's theory was widely and understandably ridiculed, for he proposed no mechanism. But long after Wegener's death his view was mostly vindicated by the now universally accepted theory of plate tectonics.
These episodes are exemplary, not just because the thinkers were brave or creative enough to challenge orthodoxy, but because they succeeded in developing a model that explains both the illusion and the real underlying phenomenon. Even most who achieve this are not ultimately successful. Plato agreed with Parmenides that the reality of the physical world is an illusion. He explained it poetically as a shadow cast by the real world of the Forms. But the shadow was a metaphor drawn from the physical world, and the perfect world of Forms turns out to exist only in our imaginations. There are many other examples of the kind I have given above, historical and contemporary, successes and failures. Perhaps our dancers will identify some of them.
G. Randolph Mayes
Department of Philosophy