It is worth spending an extra moment on the slippery slope, one of the most common of this chapter's fallacies and also one of the most common accusations made against someone else's position in an argument. Sometimes the participants in a discussion will label anything their opponents say a slippery slope, if it makes any reference to the effects of an action or belief.

Every such reference to effects, good or bad, rests on predictive claims, so you need to stay alert to predictions of what follows from what else, and what basis in fact those predictions may have.

Consider the following reasoning from a sleepy parent: "If I get up at 3:30 a.m. today to feed the baby, she'll be more likely to cry again at 3:30 tomorrow, or even 3:00, and then it will take her longer to fall back asleep." Although the parent may be exaggerating these dangers, the core prediction -- that the baby will interpret being fed at 3:30 today as an incentive to cry in the middle of the night tomorrow -- has a good deal of plausibility. So does the parent's belief that it is easier to stop a bad habit from beginning than to eradicate an existing one.

The psychology of infant behavior is much simpler than the collective psychology of an entire society, and neither element of the parent's argument applies directly to more complex questions. Claims about social tendencies, and about the difficulty of stopping them, call for more substantial evidence and analysis.

Doctor-assisted suicide provides a controversial example. Some opponents to its legalization argue on the basis of worse consequences to come. "Once we allow doctors to help people commit suicide when they clearly want to, they will provide that same assistance to patients who are incompetent to make a decision for themselves, or even try to convince undecided patients to kill themselves."

This argument makes two claims: (1) that physicians will change their behavior and their expectations from patients once a certain practice has become legal, and (2) that society will find itself unable to stop that egregious behavior. Since these are claims about fact, and debatable, they require some support.

In this particular case, those who make the argument often appeal to the example of the Netherlands, where doctor-assisted suicide is already legal. Some argue that its legality has resulted in unwanted deaths. Does this mean there is no slippery slope at work?

It depends. You should examine two matters before deciding whether to accept or reject the argument at this point: whether the claims about what happens in the Netherlands are true, and whether social circumstances present there are also present in the United States (since the consequences of a policy in one country do not always translate to another country).

What matters more to us is that this discussion has already shifted to citing and evaluating evidence. That's a sign that we have left the domain of the slippery slope. Slippery-slope arguments tend to rest on anecdotal evidence, or no evidence at all. In many cases, a wild prediction about what will happen next lets you know that someone has conjured up a slippery slope but not really thought about it.