Thankfully we’re not limited to either as a philosophy. and Intelligent Design Theory is almost always used to refer to the types of “irreducible complexity” arguments defended by William Paley, not just to the idea that their must be an intelligent creator. Hence my slightly flippant use of the ™ symbol above.
ID’s weakness: The idea of “design” is one of common sense. It is not measurable in the sense that one can draw a line that separates that which is designed and that which is not, and another line that separates that which is complex from that which is not. Having a key attribute – design – that cannot be measured and mathematically modeled, science must drop out as a strong ID proponent.
ID’s weakness is that irreducible complexity (as the argument is presented) is not something that can really be demonstrated at all, despite claims that it is scientific. And furthermore I don’t see any reason why the brain should be evidence of design and a rock, or a hydrogen atom, or an electron should not. It’s an attempt to draw arbitrary lines across a mistaken, mechanical (that is, simply mathematical modeling and mechanical parts) conception of nature.
Darwinism’s weakness: First, the mechanism claimed that works on random mutations, natural selection, cannot be verified. We have evidence of different kinds of life and evidence which indicates progression from simple to complex beings but no evidence of natural selection as the driving mechanism. “Survival of the fittest” is pure theory, rational but not empirical. Second, randomness is a perceptual claim rather than an observational claim. That is, if one cannot see the cause to effect then, in ignorance, one only perceives the effects to be random. Fair enough for now. The molecular biologists will illuminate us, hopefully, soon.
Thankfully the alternative to rejecting IDT™ is not just “Darwinism,” though the term is often thrown around as a vague pejorative. I take no issue what the model of natural selection science shows. The naturalist metaphysical conclusions/baggage some take with it, though, is hardly required. Darwinism as metaphysics is deeply flawed.
As we are in the philosophy forum, we can also size up the two theories as philosophies on the origin of species. Philosophy does not limit its inquiry to the same constraints as do the empirical sciences. But philosophy does limit the movement from observation to logical inference based on metaphysical principles. Darwinism fails as a a philosophical explanation primarily, but not exclusively, because the theory violated the principle of sufficient reason , i.e., an effect cannot have properties not present in one or more of its causes.
ID does quite well as a philosophical explanation of simple to complex beings. Taken at its most complex, ID offers the only explanation for the human mind.
This bears on some nuances which go beyond “irreducible complexity” arguments, in my view.