Reductio ab Absurdum is a criticism of a position which shows that it is false by demonstrating that absurd conclusions follow from it. A Straw Man is a fake position that sounds like someone’s real position which is constructed by an opponent because it’s easier to disprove than the person’s real position. (It is often the case that the straw man is accidentally constructed because the attacker has never understood his opponents real position.) These two are often confused for each other, which is a bit odd, and I think that a big part of the explanation for why is Kantian epistemology. (I wrote about Kant’s substitute for knowledge here, and this blog post won’t make much sense unless you read that first.)
The relevant part of Kantian epistemology is that each of the several contradictory universal theories held by a person are held only in the areas of life in which the person believes that they produce correct results. In all other aspects of life, the universal theory is ignored. To continue with the example of neck-down darwinism, survival of the fittest is not even considered in the realm of politics, and all men being created equal is not even considered in the realm of science. Each theory has its proper domain, not in the sense of the domain where it makes claims, but rather the domain where its claims are heeded. This is the key ingredient in reductio ad absurdum being called a straw man.
Suppose Fred and James are arguing, and James holds a Kantian epistemology while Fred does not. Fred points out that James’ materialism implies that no action is any more moral than another, because no human action creates or destroys matter. James says that this is a straw man, because he never said that. Yet Fred never claimed that James said that, he claimed that James would have to say that if he were being consistent with what he (James) did say. Why is James so convinced that his is a straw man?
It’s because morality is not someplace that James applies materialism. To James’ mind, showing that one of his universal theories has implications is not enough to prove that James believes those implications. Instead, it must be shown that James actually believes that the universal theory should be applied to that part of life. James see’s Frank’s reductio argument as a straw man because James does not believe his universal theory (materialism) should be applied to this part of life (morality), and so its implications in that area of life are in no way his position.
It’s difficult to know what to make of James’ contention that this is a straw man of his position. In a sense he’s right, because that implication of materialism is not his position. But that’s because materialism is not in fact his position, despite the fact that he has claimed it to be. In fact, he doesn’t actually have a position. His claim believe that truth is unknowable and so the best we can do is refining our theories as we can “test” them against evidence is basically a methodological form of blank skepticism. It makes no positive claims of any kind, other than the self-evidently true ones about what at the moment appears to be the memory of previous experiences, and as such attributing any positive claim to it is mistaken. This is an utter failure of rational thinking, but that’s really the only criticism which can be leveled against it. By claiming to no more knowledge of the world than is possessed by a worm, it cannot be proven wrong about anything. The real problem is that the people who claim to believe this are essentially committing the moral crime of stolen valor. Just as a deserter pretending to be a decorated war hero is reprehensible, so is a putative earthworm who still wants to be treated like a man. Such skeptics would be consistent enough if they didn’t complain about being treated like worms, but they do. They rely on the fact that we don’t believe them to live a much better life than they are entitled to according to their philosophy of the world. In argument, they are essentially taking advantage of good manners, because if we were to take their words seriously, the only correct response amounts to, though it is possible to state it less bluntly, “shut your mouth among your betters, dog”.