Slippery slope refers to a type of argument in which a number of premises are given, each one slowly moving closer to the desired conclusion. But because not every premise is certain, the conclusion is not as certain as the person making the argument would have you believe.
A therefore B
B therefore C
C therefore D
D therefore SOMETHING VERY, VERY BAD!!!
So, Not D. So Not C. So I guess we have to give up B too. And that means no A. Sorry.
Also called "camel's nose in the tent"
Here's a made up example:
- If we don't stop the Communists in South Vietnam, they'll take over the whole country.
- If they take over Vietnam, next they'll conquer Laos, Cambodia, and Thailand.
- Once they have Laos, Cambodia, and Thailand--then they'll overrun Indonesia and the rest of the Pacific Rim.
- Once they conquer the Pacific Rim, they'll take Japan--and the next thing you know, they'll be off the coast of California!
- implicit: Communists invading California is unacceptable.
- explicit: We must stop the Communists in South Vietnam.
Slippery slope is also used to refer to any behavior or rationalization which seems harmless at first, but will become harmful if it continues. Drug use, for example, can be done responsibly, but with increased tolerance and increased acceptance of drug use, small doses become large and 'soft' drugs become hard drugs.
Slippery slope is more likely to be used in reference to complex moral issues (war, animal testing, etc.) than drug use; it's a way of saying that once you start any behavior of a dubious type, more and more morally unsound behaviors will be easier and easier to rationalize.