This has been cropping up in the comments on my recent abortion post, mostly from a scientific perspective, with insightful remarks from bram452, bridget_coila, ellameena and others about conception, gestation and parturition. Well worth going back to read, if you didn't follow much of the comment thread on that post.
However, I'm going to shift my examples to Creationism/ID for the sake of these remarks, in an attempt to get to my point without inciting the same overlay of passion that an abortion discussion engenders.
Look at evolution. The schoolyard Creationist critique of evolution, one I remember actually being said to me as a taunt around fourth grade, is "Evolution is only a theory!" As if that were somehow a bad thing. There's an obvious flaw in this logic, obvious to anyone with the slightest bit of scientific literacy, in that it conflates the technical meaning of "theory" with the popular meaning. (There's a slightly less obvious flaw in this logic, too, which I'll get back to in a moment.) I blame the English language, personally, for providing us with such elasticity of meaning that two people using exactly the same words can talk so thoroughly past each other -- not that this problem is confined to conversations about evolution. My usual response to that sort of comment is to point out that gravity is only a theory as well. Take your leap of faith off a tall building, brother, and see what the Good Lord does for you on the way down.
The obvious flaw is a lack of scientific literacy, a confusion (deliberate or otherwise) of meaning regarding words such as "theory." This, incidentally, is one of my biggest beefs with Bible-based education, whether in the home or in the schools -- I am highly suspicious of these confusions and distortions being deliberately introduced in order to protect faith.
The slightly less obvious flaw extends to a lack of philosophical literacy, a confusion of the meaning regarding words such as "truth" and "belief." When I say "I believe that the theory of evolution is true," I'm making a statement that subsumes a large amount of empirical learning and experience -- in effect, it's shorthand for "given my training in the scientific method, my understanding of the peer reviewed research process common to modern science, and my basic knowledge of biology, genetics, paleontology and other scientific disciplines, I understand that the theory of evolution is overwhelmingly well supported by available evidence and all accepted tests for validity", or something of the kind. I can't personally prove evolution is true, but I can't personally prove gravity works either, only demonstrate. Likewise for many other aspects of the physical world -- the sun could be a giant ball of flaming peat moss, I don't have a 93 million mile long dipstick. But I do have sufficient basic grounding in astronomy and physics to be confident that the answer is otherwise, even if I can't follow the math, spectroscopy and other bits of evidence required.
So in that context, "believe" is shorthand for "have the education and understanding of the process sufficient to evaluate the proposition," while "true" is, as stated at more length above, shorthand for "the process has produced a valid proposition."
Whereas, when a Young Earth Creationist says, "I believe that it's true God created the world 6,000 years ago," they're using both those words -- "believe" and "true" -- in a completely different sense. They're talking about a mythic or spiritual truth which they have chosen to accept, not about an external proposition with any objective defensibility.
Yet our media, our schoolboards, our entire public discourse, equates spiritual truth with empirical truth. Which is why we get the kind of antiscience idiocy around evolution, global warming and other propositions that is such a strong thread in electoral politics, education policy and governance in general.
I want to say to those people, "just because you believe it doesn't mean it's true."
My own understanding and acceptance of evolution is not in some sort of balance-of-validation with their belief in Creationism. We're simply talking past each other. Intelligent Design, a specific bugbear of mine quite frankly, is merely Creationism dressed in the surface language of science, to confuse people who don't understand peer review, falsifiability and the other highly important details of scientific inquiry. It lets people stand squarely with their religious beliefs without having to feel guilty about adopting a pre-Enlightenment irrationalism. The whole "teach the controversy" meme is deliberate, cynical epistemological fraud designed to appeal to the American desire for fairness and balance.
You can believe anything you want, with my enthusiastic support. Believe in the Bible, follow the Buddha, honor hyperintelligent shades of the color blue. It makes me no never mind. But when you confuse your spiritual belief with the empirical reality of the world -- in biology, climatology, foreign policy or anything else -- you're indulging in magical thinking and pissing in the font of public discourse.
Zip it up, people, and learn to think clearly.
(And yes, I still blame the English language -- a lot of this confusion would be a much harder sell if we had a slightly more detailed ordinary vocabulary for epistemological concepts.)