"The slippery slope" is an argument that one event or decision will unleash a chain of other events or decisions leading ultimately to a highly undesirable outcome. Although it can be used in a logically valid way, in many cases it is based upon an assumed overestimate of the likelihood that each event in the series will cause the next one; in such cases, it is merely a form of guilt by association, which is a logical fallacy.
Note that the slippery slope argument often works just as well in reverse. If I can argue that "Allowing A will lead to B and then who knows what else we'll have to allow?", you could counter argue "Yes, but if we prohibit A, then we'll also have to prohibit C and then who knows what else we'll have to prohibit?" In a situation where A is already prohibited, of course, the "who knows" question is already answered; this reveals something of the argument from ignorance nature of the "slippery slope" argument: "You don't know what will happen, so it must be bad."
Logically valid usage of a slippery slope would need to either describe each step from the initial event to the final, clearly undesirable event, or else describe a process by which the initial event leads to the final event.
- from Slippery Slop, Slate magazine, 2004-05-19:
- "If the Supreme Court says that you have the right to consensual (gay) sex within your home, then you have the right to bigamy, you have the right to polygamy, you have the right to incest, you have the right to adultery. You have the right to anything." — Senator Rick Santorum
- "Once you cross that Rubicon, then there's no place to stop. Because if a judge can say two men and two women can marry, there is no reason on Earth why some judge some place is not going to say, this is not fair. Three women or three men, or five and two or five and five." — James Dobson