ID/creation vs. evolution

Once again, I'm dipping my toe in an area I ought not due to lack of education. I just wanted to respond to an argument I've heard frequently when this subject comes up.

Critics of intelligent design or creationism will often say that IDers or creationists only offer arguments against evolution. They do not provide any positive evidence for their own point of view. The critics will then say that even if evolution were false, that would not make intelligent design or creationism true.

Whether IDers/creationists do provide positive evidence for their own point of view isn't my concern right now. I just want to respond to that last statement--that disproving evolution does not prove ID or evolution.

It seems to me that there's only two possibilities--either life was engineered somehow, or life arose through purely naturalistic means with no purpose behind it. I don't see how there could be any other possibility.

If life was engineered, then some sort of intelligent design is true.

If life came about through natural unguided processes, then it either came about gradually or all of a sudden. Now I don't think anybody is going to argue that a fully formed biological entity like a cat or a human just came together naturally all of a sudden with all of its parts in place. So if cats and humans emerged through a natural process, then it happened gradually.

Maybe it is due to lack of imagination on my part, but assuming life came about gradually through a natural process, the mechanisms of evolution (mutation and natural selection) are pretty much the only game in town. So one could make the following disjunctive syllogism:

1. Either ID/creation is true, or evolution is true.
2. Evolution is not true.
3. Therefore, ID/creation is true.

If ID/creation and evolution are the only possibilities, then any argument against one is an argument for the other. The only way to escape the argument is to come up with some third possibility. If there were a third possibility, then an argument against one view would not necessarily be an argument for another view since there would be two other possibilities.

But let's suppose there is some third possibility. As I argued above, the third possibility would have to be a natural process because either life was engineered or it wasn't. That's just the law of excluded middle. So we can form this argument:

1. Either ID, evolution, or X.
2. Not evolution.
3. Therefore, either ID or X.

So an argument against evolution wouldn't necessarily be an argument for ID since X might be true instead. But it would narrow the scope, and a person could make this argument:

1. Either ID or X
2. Not X
3. Therefore, ID.

Depending on what X is, a person could still work his way around to an argument for ID. Disproving evolution would just be the first step.

But before somebody offers an X in order to disprove the ID/creation vs. evolution disjunct, he's first got to make sure that X is a viable option. I mean if it turns out that X is less likely than ID/creation, then an argument against evolution is still going to work as an argument for ID/creation. In light of arguments against evolution, X will weaken the case for ID/creation in proportion to the probability of X. If ID is far more likely than X, then an argument against evolution makes ID far more likely. If X is just as likely as ID, then an argument against evolution makes ID and X equal alternative candidates. If X is more likely than ID, then an argument against evolution makes X more likely to be true.

You don't have to subscribe to ID, creationism, evolution, or X to make the point I'm making. I'd make this same point regardless of which I thought was true. It seems pretty obvious to me.