Evolution does not necessarily exclude Adam and Eve and the Fall, and evolution is not a scientific conclusion, obvious or otherwise. For Christians to reckon with evolution they must understand evolution. And to understand evolution, they must understand IFF. Understanding IFF does not force one’s position on evolution, but it does force one’s understanding of evolution…"If and only if" (usually abbreviated "iff", but not in all caps) is an extremely elementary concept in logic. But the creationist blogger, Hunter, is treating it as an advanced concept. It's as if understanding "IFF" shows that he is intellectually superior to scientists. Instead, it just shows that elementary logic is a novel concept to Hunter.
And while this is a perfectly good use of IFF, IFF has no place in scientific hypotheses. A scientist would never say “if and only if my hypothesis is true, then we will observe a certain observation.”
The irony is compounded by the fact that Hunter does not in fact seem to understand "if and only if" and its application. It doesn't make any sense to say "IFF has no place in scientific hypotheses". "If and only if" is a logical connective, just like "and" and "or". Would it make sense to say, "AND has no place in scientific hypotheses"?
Basically, the whole discussion of "IFF" seems to be a way to dazzle the audience with logical knowledge, and mask the argument's central incoherence.
A fun comparison comes to mind: Objectivism.
Many people know Objectivism for its peculiar form of libertarianism, but it's a comprehensive philosophy which also covers epistemology and metaphysics. Objectivism begins with three axioms: existence exists, conscious exists, and the axiom of identity ("A is A").
I find it strange that a philosophy would claim to be based on only a few axioms (and pick such terrible axioms too). It's clearly trying to emulate the structure of mathematics. But it's not really succeeding, because math doesn't really assume that its axioms are true, it just constructs useful or interesting axiomatic systems, and tries to find tautologies within those systems. Furthermore, it's not clear that the axiomatic structure of mathematics appropriate here; certainly science doesn't use an axiomatic structure.
The most absurd axiom is "A is A". It just doesn't belong. Why take the axiom of identity, and not the axiom of symmetry ("if A is B, then B is A") or the axiom of transitivity ("If A is B and B is C, then A is C")? I think it's just there to make it sound math-y, and therefore solidly correct. I can't imagine a legitimate use the axiom of identity in any argument over Objectivism, unless the construction of natural numbers is one of their substantive issues.
Commenter Larry once explained this by saying Objectivism fetishizes deductive reasoning. That sounds about right, but I'm uncomfortable with using "fetish" as a pejorative, so I found "infatuation" in the thesaurus.
Objectivism is infatuated with deductive logic, and wants to emulate math even when it is foolhardy and inappropriate. The creationist Hunter is also infatuated with logic, and makes a big deal out of a logical connective at the expense of coherence. There are some differences too. For one thing, I believe most Objectivists, unlike Hunter, at least understand the deductive logic that they are infatuated with.
I'm not going to overextend my comparison, since I don't believe Objectivists and Creationists are particularly similar groups. I was just wanted to talk about two examples.