My friend, Richard Mouw, a philosopher and former president of Fuller Theological Seminary, has raised an important challenge about the use of counterexamples when making one’s case on certain controversial moral and political questions.
He shares one of the arguments he employs to explain to his friends why he opposes the legal recognition of same-sex “marriage” (SSM): “If we are to operate as a society on the assumption that any sincerely held view about what constitutes a marriage should be granted status in our laws and practices, I have asked, what would keep us from legalizing plural marriages, or even incestuous ones?” Mouw says that his question is often “met with disdain,” with the retort, “[C]an’t you do better than a ‘slippery slope’ argument?”He finds the retort frustrating, since, “some slopes are indeed slippery, and we do well to approach them with caution.” In other words, if you advance the truth of principle X in order to justify practice Y, something that you support, why not also accept practice Z, something that you reject, since it too is entailed by principle X?
So, for example, if you support the legalization of marijuana for competent adults on the principle that “one has a right to do whatever one wants to one’s body without directly harming others,” then that principle not only justifies marijuana legalization but also the decriminalization of hard drugs like heroin.
A person who resists this entailment by saying it’s a slippery-slope fallacy is confusing the fallacious form (often called a “causal slippery slope”) with the legitimate form of the slippery slope (often called a “logical slippery slope”). To point out that a principle entails something undesirable is not a slippery-slope fallacy. It’s an acknowledgment that principles have a logic of their own, so to speak.