One of the inevitable debate subjects between evolutionists and creationists is whether or not creationism or evolution is scientific. In order to understand that debate, we must understand what “science” really is.
Much of what we call science in the 21st century is based on the work of scientific philosophers. Probably the most influential of them was Karl Popper. Here’s how Wikipedia describes his philosophy of science:
"Popper argued strongly … that scientific theories are abstract in nature, and can be tested only indirectly, by reference to their implications. He also held that scientific theory, and human knowledge generally, is irreducibly conjectural or hypothetical, and is generated by the creative imagination in order to solve problems that have arisen in specific historico-cultural settings. Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single counterexample is logically decisive: it shows the theory, from which the implication is derived, to be false. Popper's account of the logical asymmetry between verification and falsifiability lies at the heart of his philosophy of science."
In summary, a scientific theory can never be “proven”; proofs are reserved for mathematics. Instead a truly scientific theory is only subject to falsification.
Furthermore each and every scientific theory must, by its nature, suggest some possible test that could potentially falsify that theory.
A classic example of such suggested falsification was given by Alfred Einstein when he proposed the General Theory of Relativity. General relativity was based on the presumption that space itself was curved by gravity. Einstein suggested that this idea could be tested by seeing if light from distant stars would be bent by the sun’s gravitational field. Then, in 1919 during a total solar eclipse, a scientist named Arthur Stanley Eddington was able to confirm Einstein’s prediction. If the light had not bent then Einstein’s idea would have been falsified.
Evolution is scientific because it could be potentially falsified. In fact, there are many possible things that could falsify evolution. One of the more interesting ones would be the existence of an animal such as the winged horse Pegasus. Evolution depends on there being a chronological progression of characteristics. “A Pegasus would be a mammal closely related to the horse, its wings would be considered derived characteristics. However, Pegasus wings cannot be modifications of its ancestors' structures, since the immediate predicted ancestors of Pegasi (horses or the ancestors of horses) had no possible structures there to modify”. So the existence of such an animal, or even the fossil of such an animal, would falsify evolution. (I discuss this example – called a chimera - again later in this book.)
One of the strongest arguments in favor of evolution provides another possible way of falsifying it. That argument is biogeography. Certain species are found only in specific locations; often they are found in very specific locations. Such species should only be found close to where their ancestors lived. This is particularly true of recently evolved organisms. Because of that, “We confidently predict that fossils of recently evolved animals like apes and elephants should never be found on South America, Antarctica, or Australia (excepting, of course, the apes that travel by boat)”.
Creationism can present no similar possible potential piece of evidence that would falsify its ideas. (Or at least creationists refuse to acknowledge evidence that indeed does falsify their ideas.) Ask a creationist for a specific piece of possible evidence which, if found, would falsify their hypotheses and you will not get an answer, or at least you will not a specific or realistic answer. In my debates with creationists I’ve been told that creationism would be falsified if examples of every single transitional fossil could be found (i.e. if all living things had been fossilized and their fossils could be found and sorted). I’ve also been told that creationism would be falsified if someone could demonstrate humans evolving from apes in a laboratory (a process that, in fact, took millions of years in nature). But specific, realistic examples are never given.
That’s why creationism is not science and why evolution is.
I should mention that non-creationists believe that the Biblical accounts have been falsified. The story of the flood of Noah in the Bible offers many opportunities for falsification which indeed do falsify that account.
But creationists will simply insist that God performed this miracle or that miracle to “explain” away those problems. The use of miracles to explain away problems prevents calling creationism a science.
 http://en.wikipedia.org/wiki/Karl_Popper, referenced on June 5, 2008
 Futuyma, D. (1998) Evolutionary Biology. Third Edition. Sunderland, MA, Sinauer Associates, p. 110
 http://www.talkorigins.org/faqs/comdesc/section2.html, referenced on June 19, 2008