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the sun because the light would be bent by the gravitational field around the sun to be visible on earth. And, with reference to criterion number one this was a testable claim. Once tested, it was verified.

Third, a theory should have a wide scope. That is, a good theory explains a wide field of evidence. One of the differences between theories and hypotheses is their scope. Hypotheses address specific questions whereas theories attempt to provide a broad explanatory device. Theories that can explain a wide array of things are preferred, other things being equal, to more narrow theories.

Fourth, a theory should be simple. This term should not be confused with simplistic. Many scientific theories are complex in terms of our ability to understand them but simple in the sense that they postulate fewer underlying entities or assumptions. A good example is the difference between Copernicus and Ptolemy. Ptolemy's geocentric theory could explain the orbits of the planets but it was quite complex where as Copernicus' theory explained the same observable phenomena with less complexity. So, other things being equal, that theory was the better theory. Think of it this way. Suppose I come up with a theory to explain how the lights in my house work but it involves little gremlins running inside the light bulbs. Someone else is able to explain the same phenomenon but without postulating gremlins. So, their theory is simpler than mine. It should also be pointed out that my gremlin theory may fail on other criteria as well such as being testable.

Finally, a theory should be conservative. Not in the political sense of the word. Rather, it should fit in with other things we know. If we have an explanation for something that we think is fairly certain and accurate then a new theory should fit in with that prior explanation. If it doesn't fit that may indicate our prior knowledge is flawed. We have to be open to that possibility but the burden of proof is on the new theory. An interesting examination of how this process works is offered by Thomas Kuhn's book The Structure of Scientific Revolutions.

Now, for the payoff. While many may hate to hear this, judging by the criteria above, the theory of evolution and the big bang theory do quite well. In fact, as the philosopher Daniel Dennett puts it "evolution is about as well established as the fact that water is H2O." This doesn't mean that the theory isn't open to revision. But, all available evidence seems to point in favor of it.

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