A key point I emphasize in my classes is that, just because a range of views about a topic may be held by experts, that does not mean that all views are compatible with the evidence. Nor is it the case that, just because we are uncertain about the answers to some questions, everything is uncertain. The above image featuring a quote from Jerry Coyne makes the point well, I think, in relation to one specific topic about which denialists try to use unanswered questions to deny the accurate and well-evidenced answers we do have to some questions. But the general point is applicable as well to history (Coyne would do well to take that to heart) and other areas.Phil Plait also made some important points with admirable succinctness and clarity on his blog today:
The difficulties in debunking blatant antireality are legion. You can make up any old nonsense and state it in a few seconds, but it takes much longer to show why it’s wrong and how things really are.
This is coupled with how sticky bunk can be. Once uttered, it’s out there, bootstrapping its own reality, getting repeated by the usual suspects.
See also Irtiqa on crazy conspiracy theories.