There are numerous Web sites (not to mention books and other publications) that explain in various levels of detail what scientific theorizing is all about. What I am about to say is, I believe, consistent with what scientists themselves have to say about it. Those who are so inclined may wish to check out one or more of the following, or put their own search engines to work.
http://www.astronomynotes.com/scimethd/s2.htm
http://atheism.about.com/library/FAQs/evo/blfaq_sci_theory.htm
http://www.johnpratt.com/items/astronomy/science.html
A theory is a testable, predictive, and logically valid explanation for some set of observed facts. The evidence for any theory is the body of facts that it purports to explain. That is to say, scientists in effect are claiming, "We would not observe these facts unless the theory was true."
There can be, and in the history of science there often have been, competing theories purporting to explain the same facts. The scientific method is, among other things, a way of determining which of two or more competing theories provides the best explanation, i.e., is most likely to be a true explanation of why we observe what we observe. For now, though, we'll just look at what it means, to a scientific thinker, for a theory to be scientific, by examining the qualifiers I gave: testable, predictive,and logically valid.
Testable: The theory has to be stated in such a way that it could, at least in principle, be proven wrong. Another way to say this is that there ought to be some observable difference, logically implied by the theory itself, between a universe in which the theory was true and a universe in which it was wrong.
Consider the theory that grass is green because it gets painted every night by invisible elves using methods undetectable to scientists. Now, there are lots of reasons why the conventional theory is better, but leaving that aside, how would we know if the "elf theory" was wrong? A believer would say, "Grass would not be green if it weren't for the elves." Yes, but what logic compels us to think so? The elf theory says, in effect, that what the elves do is not observably distinguishable from what the conventional scientific theory says is happening. It is therefore not truly a theory in the scientific sense.
Predictive: This is related to testability. A theory must logically imply certain things about observations not yet made. Besides accounting for things already observed, it must lead us to expect certain things not yet observed.
Scientists used to have an ether theory to explain the propagation of light through space. The hypothetical ether was something like an atmosphere that filled the entire universe. We need not get into why they thought the ether had to exist, but its existence did seem logically necessary. Light had been observed to do certain things that seemed, at the time, to be inexplicable without there being something like the ether.
Well, the theory said, if the ether is real, then if you measure the speed of light under such-and-such conditions, you'll get such-and-such results. So, in 1887, Albert Michelson and Edward Morley measured the speed of light under those conditions, and they did not get those results. Well, that was just too bad for the ether theory. The scientific community had to come up with another way to explain the observations that the ether theory had tried to explain. And they did.
If a theory is formulated in such a way that any observation at all would be consistent with it, then it makes no real predictions and is therefore not a scientific theory. A great deal of theorizing about ESP is like this. It basically says that sometimes it works and sometimes it doesn't - which would be OK, except that the believers can never tell you when it is supposed to work and when it is not - and the observed patterns of when it works and when it doesn't is indistinguishable from what we would expect to see if ESP were not real. In other words, "ESP theory," such as it is, predicts nothing different from what we would expect to observe if it were false.
Logically valid: A theory must establish a logical relationship between its suppositions and the observations it explains, and the logic must be consistent not only within the theory but also with other accepted theories. If a new theory is found to be internally inconsistent, then it is rejected out of hand. If it is internally consistent but contradicts some previously established theory, then either the old theory or the new theory has to be adjusted to make them consistent.
Next: What does the theory of evolution really say?
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