In chatrooms and creationism websites the claim is often made that evolution is "only a theory" or that it is not, and should not be taught as, a "fact." Most often these claims arise out of the need for fundamentalist believers to defend non-negotiable elements of faith. Semantical arguments are also made by students recently introduced to elementary courses in philosophy and logic. In other cases questions are raised by persons honestly unaware of the definitions and methods of science.
In an age where sound bites, opinion polls and glancing at magazine covers replaces reading and programs for continuing education, it is perhaps understandable why the majority of people are woefully ignorant of science generally, and of its definitions, specifically. For those who wish to deny evolution and the findings of science it is easy to find in a dictionary a definition for a word to assist their preconceptions. But dictionaries are not arbiters of science. Scientific definitions are the product of consensus among working scientists and editors of peer-reviewed journals of science. In discussing science we should speak her language.
Words such as fact, hypothesis, theory, law, proof, probability, true and false have meanings within science. Dictionaries often do give adequate, brief, scientific definitions for these words. But it is often necessary to understand those definitions in a broader context (often given in specialty dictionaries of science).
I present below a few explanations given by eminent scientists. Below those you will find a few links to other webpages that provide alternate definitions and viewpoints. If you remain puzzled after reading thoroughly these explanations and definitions, my email address may be found at the very bottom of this page.
Scientific hypotheses are, in one way or another, tested against nature -- the "real world" that all scientists conventionally agree is "out there."1 Only when hypotheses are sufficiently tested and bind together information from relatively diverse areas that previously had not been connected do they properly become theories. But that is the opposite of the popular understanding of the term; it's scientific meaning is much closer to that of the word fact in common parlance.2 Theories embody the highest level of certainty for comprehensive ideas in science. Thus, when someone claims that evolution is "only a theory," it's roughly equivalent to saying that the proposition that the Earth circles the sun rather than vice versa is "only a theory." Evolution is, in fact, a very useful theory. [underscore added]
If the extreme version of solipsism (the notion that only the self and the contents of one's own consciousness exist -- or can be known) were taken as correct, one could not do science.
A scientific theory can be defined as a coherent framework for an entire field, one ordinarily so well supported by empirical data that scientists act as if it were "proven." Once established, theories are treated as "true" (I use quotation marks because truth itself is not a scientific concept; all scientific ideas are subject to revision) as long as they are useful -- generally until and unless some scientist demonstrates them to be wrong. Any position that is based on authority and is not subject to revision cannot be accurately described as scientific.
In the American vernacular, "theory" often means "imperfect fact" - part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is "only" a theory and intense debate now rages about many aspects of the theory.
Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them.
Moreover, "fact" doesn't mean "absolute certainty"; there ain't no such animal in an exciting and complex world. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are NOT about the empirical world. Evolutionists make no claim for perpetual truth. In science "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional consent." [underscore added]
Evolutionists have been very clear about this distinction of fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory - natural selection - to explain the mechanism of evolution.