Slippery slope style arguments tend to be fallacious. In one sense, they suggest non-existent causal chains as reasons not to engage in some or other activity. An example: If we allow gay marriage in California, then we will have to allow polyandry, polyzoology, poly-whatever-you-wish, because the door will have been opened, the foot will be in it, and the slope will be greased and increasing its degree of descent. Downward, indeed, we will go if we allow gay marriage. That of course is not so much an argument against gay marriage as it is an argument against the things that would follow gay marriage. Perhaps it's an implicit admission that one has no argument at all against gay marriage. This causal chain, of course, starts at straight marriage. Seems like if we allow that, then we will have to allow marriage between to "straight" Christians, and then therefore etc., as they used to say. This of course points to the other variety of slippery slope fallacy–the relevance variety. Man-turtle marriage really isn't what one was talking about when one advocated gay marriage. Man-turtle marriage is irrelevant. It's not like man-man or woman-woman marriage at all. In the first place, turtles can't contract. So there's that.
Sometimes however slippery slopes are not fallacious. No, these are not the slippery slope arguments that I use–because, as we all know, I can never be guilty of a fallacy. Rather, these are slippery slopes that aren't causal, but rather analogical. If we make a law, for instance, which benefits company A, call it, I don't know, GM, then, by analogy, we must also make a law which benefits company B, which finds itself in the same circumstances. That's not really a slippery slope in the fallacious sense, as it's more of an argument by analogy anyway.
I make this point because I encountered this surprising instance of a non-fallacious argument in Michael Gerson's piece today. Speaking of a government bailout of General Motors, he writes:
But wouldn't government intervention be a slippery slope? Why bail out GM and not Circuit City? Well, perhaps because the closing of Circuit City leaves an empty place at the mall, while the collapse of the American auto industry would leave entire regions of the country in crisis. It is the job of a president — on issues from military intervention to economic policy — to keep his footing on unavoidably slippery slopes.
Maybe there is also a kind of implicit false dichotomy here as well–one can either help everyone or no one. There is no middle ground. We cannot afford to help everyone. So we must therefore help no one. Or maybe perhaps there's a kind of fallacy of accident–the misapplied general rule: if the rule states we help companies who are in dire straits, then we must help all companies regardless, etc. That's what the rule states, after all.
So Gerson is of course right that is not a slippery slope. But he's wrong as to why. The reason why it isn't is because not all slippery slopes are fallacious. One sometimes hears complaints in the fallacy literature to the effect that some alleged fallacies are not fallacies at all. My answer to that is a resounding "maybe" or "sometimes." Sometimes they're not. Sometimes they are. When they are, they're fallacious.