On an old post at Soft Machines, commenter Hal wrote:
One big problem I have arguing with pro-Drexlerites is the issue of “burden of proof”. They often argue, as Brian Wang does two posts above, that there are a lot of ideas that might work, and it is up to opponents to come up with exhaustive proofs that the technology will fail. I have tried to point out that given the radical and extraordinary claims for this technology (human immortality, and machines that build anything instantly that you wish for, among others), the burden of proof needs to be on the proponents of the technology to show that it will actually work. Until that is done the appropriate response is, we’ll see.
This is exactly wrong. When a technology is claimed to fulfill radical goals, it is worth checking whether that is indeed what the technology, if invented, would do. But if we are trying to find out whether we can invent the technology, by debating physical chemistry and the like, then radical consequences are irrelevant. Nature does not have a built-in anti-science-fiction censor.
Although there may well be reasons to be skeptical of Drexlerian nanotech, this is not one of them, and I suspect the error is common.