It was gratifying to see Jon Stewart visit the topic of logic on last Thursday’s Daily Show. I would have blogged about it sooner if I could have. Logic is a topic we too often ignore in this country; yet a little attention to it could enormously improve our public political discourse.
The context of Stewart’s foray into logic was as follows. Republican Senator Lindsey Graham gave a common conservative argument against gay marriage, that it would open the door to three-person marriage, person-animal marriage, and things like that. Stewart accused Graham of committing the slippery slope fallacy (a common reasoning error). Graham’s friends countered that his argument was an argumentum reductio ad absurdum, a valid argument form highly respected in mathematics and philosophy. Despite all that impressive Latin, Stewart won this round of amateur logic hour, not only in the eyes of his audience, but really.
Graham’s friends are right that his argument can be put in reductio ad absurdum form. As impressive as that sounds, it’s not. Although many great, important arguments take that form, so can any number of stupid, unsound arguments. The problem with Graham’s argument is not its form but its content. It absolutely depends on the claim that
If we should allow gay marriage, then we should allow three-person marriage, person-animal marriage, etc.It is this claim that Stewart accuses of the slippery slope fallacy, and he makes a fair case for that. Not as good as could be made; but not bad. He points out that some people are born so that their equal right to true happiness depends on their ability to gay marry; but no one is born so that his equal right to true happiness depends on his ability to three-person marry or person-animal marry.
The slippery slope fallacy occurs when someone argues that some apparently acceptable claim or policy must lead inexorably towards some unacceptable one, when it is not true that it must. A classic example is the NRA favorite: if we restrict private possession of semi-automatic military weapons with thirty-round clips, then next the government will take away our ordinary hunting rifles and shotguns, and the right to bear arms will disappear altogether. The false idea behind this is that there is no principled way to justify restricting private possession of any guns without banning private possession of all guns. Of course there is. We already do it. You can’t buy an ICBM but you can buy a Beretta 9mm pistol. We can draw the line anywhere we like. For instance, we can draw it where Senator Diane Feinstein has always proposed, requiring background checks for all gun sales and banning semi-automatic long-arms with folding stocks and magazines that hold more than ten rounds. I would go farther than that; but Feinstein’s view represents a principled compromise that proves the NRA’s favorite argument is a slippery slope fallacy.
(Incidentally, If we had kept Feinstein’s assault rifle ban, which George W. Bush allowed to expire in 2004, at the NRA’s insistence, and Presidential Candidate Barach Obama promised in 2008 to reinstate if he became president, bud didn’t, then apparently there would have been fewer dead elementary school children in Newtown, CT on Friday. Also, it seems worth mentioning that eight children die from gun violence every day in America. What was special about Friday was only that we got two and one-half days worth of children shot dead in just a few minutes.)
Although Jon Stewart deserves a higher grade in introductory logic than Lindsey Graham, his case that Graham commits the slippery slope fallacy can be improved upon. I doubt that anyone’s true happiness depends on his ability to three-person marry or person-animal marry; but I do not know that for certain. What I can say for certain is that animals cannot give informed consent to marry. That is a principled difference that stops the second part of Graham’s slippery slope. It may also stop the first. I doubt that all parties can be said to frequently give full consent to polygamous marriages. Even if all parties do sometimes give full consent in polygamous marriages, they claim an unequal right. The claim now before us is that same-sex couples deserve an equal right to marry with opposite-sex couples. That claim is more equal, since it involves couples in every case, than the claims of polygamists. Whether it is ultimately a strong claim depends on the case against polygamy, not the case against same-sex marriage.
Returning for a moment to the NRA and the gun lobby, I cannot be the only one who has noticed that some of them go out of their way to try to redefine what is a hunting weapon. Although there are still successful hunters who use bows and arrows or Kentucky long rifles, the NRA faithful go out hunting with modified handguns and assault weapons, for the sole purpose of proving that those are hunting weapons because they can hunt with them. Of course those weapons work for hunting. When I was in the Army I saw deer and wild cows hit, and turned inside out, by 120mm sabot rounds shot from the main guns of M1 Abram tanks, and then vultures shot off of the deer or cow carcasses with the tanks’ 240 coaxial machine guns . That you can kill animals with a weapon is not proof that it is a hunting weapon. I wonder if those same NRA folks who go out to kill deer with handguns and assault rifles will now start taking multiple wives, or marrying animals, in order to defend Graham’s stupid slippery-slope argument against gay marriage.