"Partial-birth abortion bans are not themselves that bad. But you should oppose them because, if they are enacted, much broader bans on abortion will become more likely." "Letting dying people cut off their lifesaving treatment may seem proper on its own. But if we allow that, it may lead to dying people getting help in actively killing themselves, and then over time to involuntary killing of the comatose or even of the disabled." "Embryonic stem cell research might be OK in itself, but it may lead to people getting pregnant just to get abortions." Such arguments are commonplace in debates on many ethical topics: abortion, euthanasia, genetic engineering, gun control, free speech, privacy, and more.

All these arguments express concern about the slippery slope: the risk that implementing a seemingly modest and worthwhile decision A now will increase the likelihood of a much broader...