On the sidelines of religious debate today are the topics of Evolution versus Creationism of whatever sort the proposer supports; the ongoing debate about dualism (are we our brains, or is there some illdefined additional 'spirit' or 'soul' providing consciousness?); and other topics of the like. One common thread seems to be that, among many theists, those things which are incompatible with their worldview must be wrong, because two incompatible things can't be true at once.
In fact, it's interesting to notice that, even as evangelists present arguments that are circular, plead specially, or are otherwise logically incoherent, they strive for a coherent worldview, even if at a subconscious level. After all, what is to stop a theist from accepting both evolution and creationism, and claiming that creationism actually happened but evolution is a useful framework to describe the world as we see it? I've heard at least one theist claim this, but any fundamentalist would probably reject this out of hand -- the Bible is either true or not true, it can't be both.
What makes a worldview coherent? What causes Evolution or materialism to be incompatible with creationism and dualism? What is the underlying principle here -- why is it that people seem unable to accept an incoherent worldview?
I think the most succint definition of "coherence" is a worldview where all propositions of that worldview can be held without contradiction. A contradiction arises, of course when it can be stated that "not p and p" for some p. Humans seem fundamentally incapable of accepting such a proposition, which even children intuitively realize can't possibly be correct -- and even evangelists proposing entirely asinine arguments (I had one theist claim that God could both create a rock so big that he can't lift it, and lift that rock*.) seem to subconsciously desire their worldview to be coherent (although, also interestingly, many such evangelists will accept incoherence before they will deny their original propositions).
I think turning to a specific example would now be in order. So why is Evolution incompatible with a (literal) view of Creationism? Years ago, when I was still a Christian myself, I devised an argument I termed the argument from divine similarity. Creationists state that we are made in God's image. Taken literally by many, this means that, of course, we have four limbs and are bilaterally symmetrical precisely because God shares these qualities**. The second common assertion, found especially in the new testament (although the sentiment seems to be all over the place) is that God is the same "yesterday, today, and forever." The implication, obviously, is that God does not change. If we are made in His image, and He does not change, we must not change, either. Thus, Evolution, which states quite directly that we *did* change and *do* change, is at odds with the doctrine of creationism. Either evolution is incorrect, or creationism is incorrect, because change and permanence cannot coexist. In logic, this is of course, a fine example of a reductio ad absurdum, and would be a sound argument if not for the false premise of biblical literalism.
Of course such an argument takes as its fundamental premise that the biblical passages referenced are literally true, which is a difficult premise to swallow, and one that I don't accept today. But of note is the original thought process -- there is a contradiction between not changing (as per the two passages of scripture) and changing (as per evolutionary biology). That is a contradiction which I simply couldn't swallow. I still can't, but which of the two I reject has changed. The uproar by the religious right doesn't accept this contradiction, either, but as a result they reject evolution instead of creation. Interesting, isn't it?
This leads to the concept of compatibility. While it is clear that some ideas are simply incompatible with each other, regardless of how much you may twist and contort them, what exactly does it mean for ideas at the core of our current society to be incompatible with one another? How can we decide between incompatible ideas that are equally appealing, equally popular, and equally entrenched in the minds of the public, leaving one to the scrapyard of history, and keeping the other until the next big idea comes around?
It seems to me that in all cases where any facts are in question, the answer is science, or rather, the scientific method and scientific skepticism. Incompatible ideas cannot both be correct, at least if we assume that the universe is logically coherent. So, if you have two mutually incompatible ideas, one will be supported by evidence, and the other will not.
As for the Evolution-creation debacle, well, Evolution won out. The evidence is in strong support of Evolution. For other, related debates, such as whether to allow or criminalize abortions and when, in relation to the gestation term, they should be permitted; whether climate change is happening and what (if anything) should be done about it; or, going far enough back in time, whether the sun or the earth is at the center of the solar system.
Science speaks to these questions, in most cases entirely answering the question at hand, and in others providing a solid basis for value judgments and moral arguments that require more than mere facts to conclude. So why aren't we teaching Science properly and comprehensively in the public schools, demanding the highest standard of journalistic integrity from our newscasters, or citing research when we talk about abortion? I suppose that's just one more incompatible idea to be dealt with.