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Republican leaders have become fond of denying well-supported facts in the last few years, including such well-confirmed phenomena as (at least partly human-induced) climate change, the proven safety of vaccines, and of course evolutionary theory. What seems to be guiding them, at least unconsciously, is (roughly) the following form of argument, known to logicians as modus tollens:

1. If P then Q
2. Not-Q
3. So Not-P

This pattern of reasoning is technically valid. That is, if the premises are true, then the conclusion must be true. So criticizing any argument of this form typically involves arguing that at least one of the premises is false.

For instance, here’s the Republican modus tollens argument against climate change:

1. If climate change is occurring, then we should regulate CO2.
2. We shouldn’t regulate CO2.
3. So climate change isn’t actually occurring.

Or, similarly, in the case of the vaccine Michele Bachmann doesn’t like:

1. If the HPV vaccine is safe, then we should encourage its use.
2. We shouldn’t encourage its use.
3. So the HPV vaccine is not safe.

Or, finally:

1. If evolutionary theory is true, then the biblical story of creation should be denied.
2. The biblical story of creation should not be denied.
3. So evolutionary theory is not true.

Notice that in each of these arguments, P is a matter of well-confirmed scientific theory (there are no absolutely proven theories, of course - scientific methodology rules out such dogmatic certainty), while Q is a matter of social or political policy (or, in the last argument, religious faith). This makes the conditional (“If…then…”) premises extremely problematic (even if you agree with them), because they too simplistically suggest that facts logically entail policies, rather than simply bear on them. Thanks to this oversimplification, the arguer may think that it is necessary to deny a well-supported fact in order to avoid implementing a policy or contradicting a tenet of faith. This is psychologically understandable, but irrational and unnecessary. Policy recommendations and tenets of faith can certainly be debated on their own practical (or theological) merits. We can also debate the degree to which particular facts should bear on particular policies. But factual statements should stand or fall according to the evidence for or against them, not according to the policies to which they might lead or the theological issues they may raise.

“Let your moral argument stand on its own, and let the facts stand on their own.”For instance, at the risk of incurring some cognitive dissonance, Republicans could certainly continue to oppose regulating CO2 while accepting that climate change is occurring. After all, they could argue that despite the negative environmental effects of climate change, the negative economic effects of regulating CO2 would be even worse. This might well be false, but asserting it would not be as irrational as denying climate change merely for economic policy reasons. The same sort of criticism applies to the second argument. There is no need to deny a vaccine’s proven safety record merely because you want to discourage its use for, say, moral reasons. Let your moral argument stand on its own, and let the facts stand on their own. As for the third argument, many Christian theologians - both Catholic and Protestant - deny the first premise, since they argue that the book of Genesis was never meant to be a scientific description of how people came to exist. Atheists and agnostics might prefer to deny the second premise, but what is clear is that the evidence for evolution should stand (or fall) on its own.

What is truly disturbing is that so many Republican presidential candidates don’t seem to understand such obvious points, or, if they do, ignore them for the sake of influencing the gullible.