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© Copyright 1999, Jim Loy
Occam's (or Ockham's) Razor (named after William of Occam) is the concept that when two competing ideas seem to explain the facts, the simpler is likely to be the true idea.
It should be added that this simpler idea is not necessarily true. It is just preferred until more data can clarify the situation. Also, the more bizarre and complicated an idea is, the less likely it is to be the true explanation.
Let us theorize that a mirror is a window into an alternative backward universe. When I look in the mirror I see someone who looks a lot like me (including that hair on my chin that my electric razor missed). And he seems to anticipate and mimic everything that I do. He seems to talk, but I don't hear him. He seems to live in an apartment like mine (the slob), except backwards. If I take my mirror on a trip around the world, I would see that his world is as complex and amazing as mine. And my theory becomes more and more complicated and bizarre. The alternative theory is that a mirror reflects light. This theory is complicated in some ways, but is vastly less complicated than my window into an alternative universe theory. Reflected light is arguably not bizarre, as it is the standard theory. My point is that my mirror into a backward universe cannot be disproved, from a mathematical point of view of proof. But Occam's Razor says that it is very unlikely. We choose reflected light as our explanation.
Much of science is that way. We reject bizarre theories like flat earth, astrology, cold fusion, and others, because there are simpler explanations supported by the evidence. There is much evidence against astrology. But astrology can be distorted in any number of ways to encompass the evidence. For example, sun sign predictions may only be true when you don't examine them statistically. Then astrology would become even more bizarre than it was already.
Let's look at Ptolemy's earth-centered universe. Before Copernicus, Ptolemy's system of epicycles had become very complicated and bizarre. Copernicus' theory was oversimplified (circular orbits, on philosophical grounds) and needed much tuning. But Ptolemy's theory was dissatisfying because of its complexity. Add some telescopic observations which seem to contradict an earth-centered universe, like Mercury actually going behind the sun, and Ptolemy's theory starts to get hopelessly bizarre. Nowadays the weight of evidence is even more on the side of Copernicus and his intellectual descendants. But an earth-centered universe can still not be disproved. The idea has merely become ridiculous.
Notice also that Copernicus' system of circular orbits is simpler than Newton's ellipses due to gravity and Einstein's warped space. But his theory had to be made more complicated because it did not fit the data in its simplified form.