Matthew Yglesias praises Eugene Volokh’s excellent article on slippery slopes. But oddly, he still characterizes slippery slopes as a form of logical fallacy. “It's quite true, as people say, that the ‘slippery slope’ argument is a kind of logical fallacy. Nevertheless, it is often empirically true that a slight change in one direction leads to further change.”
Perhaps I’m focusing excessively on the definition of “fallacy,” but I think Yglesias is missing a major point of Eugene’s article – a point further developed in Mario Rizzo’s and my later article on the same subject (warning: Lexis-Nexis access required). A fallacy is an argument that is false, mistaken, or invalid. There is nothing inherently invalid about slippery slope arguments, just as there is nothing inherently invalid about (say) arguments that rely on judgments of probability. The argument can be valid or not, depending on the steps in the argument and the empirical claims made. When slippery slopes are indeed fallacious, it is most often because they include a non sequitur – that is, they fail to specify the mechanism or process that will allegedly lead from the initial decision to the “danger case.” But Eugene’s article presents several slope mechanisms (indeed, the word ‘mechanisms’ appears in the title), and Mario’s and my article specifies at least four slope processes.
One of the primary reasons that slippery slopes do occur is that decision-making often takes place in a social context. The current decision-maker is not always identical to the future decision-maker: future legal cases are decided by different judges, elections change the composition of legislatures, and so on. It is therefore unwise for a current decision-maker to think, “I’ll simply do what I think is the right thing now, and then I’ll just resist doing the wrong thing later.” Even if the current decision-maker could indeed resist the temptation to make a bad decision later, other (future) decision-makers might not be so resistant. A wise current decision-maker will therefore consider what effect her decisions now are likely to have on future decision-makers. The truly fallacious argument is one that asserts – without support – that there is not, in fact, a connection between present and future decisions in a social context.
To understand the slippery slope argument, you need to see it as a meta-argument. The maker of the argument claims that the acceptance of Argument A in the present case will make the acceptance of Argument B – which may be made in subsequent cases – more likely. Thus, a slippery slope argument is an argument about arguments. Inherent in the slippery slope argument is a claim to know something about people’s ideas, and how their ideas change in response to their environment (including both economic incentives and the ideas of others). Now, it’s certainly possible to be wrong about what ideas people hold and how their ideas are likely to change, but it’s also quite possible to be right. Characterizing all arguments of this general form as “fallacious” does the valid ones a disservice.