Here’s the short answer to why ID is not science: Science concerns itself with facts that are empirically provable and empirically falsifiable. Intelligent Design theory posits a supernatural designer, whose existence cannot be empirically proved or empirically falsified. No scientific observations will ever settle the question of the existence of God. That is a question for philosophers and theologians to handle.
The hypothesis of irreducible complexity is a crucial component of Intelligent Design theory. Its proponents claim that complex organisms could not possibly have evolved through natural mechanisms, without the benefit of intelligent design. This negative claim may be true, although it is far from being proven. (In fact, some prominent examples of ”irreducible complexity” have already been ”reduced” by the natural explanations of evolutionary biologists.)
Could we ever empirically prove the hypothesis of irreducible complexity? Could we ever have empirical evidence to support the claim that no natural explanation is possible for evolution? I’m skeptical, because there will always be the danger of confusing our lack of understanding with objective impossibility. In other words, there will always be a chance that we just haven’t yet figured out the specific natural mechanisms of evolution–we can never be entirely certain that such mechanisms are impossible.
Of course, this doesn’t mean that the Darwinian theory of evolution couldn’t be falsified. It could easily be: Find a fossil of a eukaryotic cell that’s 3 billion years old, or find a mammalian skeleton that was fossilized 2 billion years ago, without any plausible evolutionary predecessors. That would throw modern biology into explanatory chaos.
But most IDers don’t seem interested in that type of activity–the gathering of empirical evidence that could falisify the Darwinian hypothesis. Instead, many of them resort to pure “God of the gaps” reasoning: They point out that we do not yet have a complete understanding of how humans evolved naturally, and they then hastily conclude that God must have designed human life. Not only is this unscientific. It’s just plain sloppy thinking.