Also called "The Camel's Nose", Fallacy of the Beard.
The slippery slope is an illegitimate use of an if-then syllogism
(modus ponens): If p then q; p, therefore q, when the first hypothetical premise p
has not been proven to be correct.
A person asserts that some event must inevitably follow from another without any argument for the inevitability of the event in question. In most cases, there are a series of steps or gradations (of the "if-then" type) between one event and the one in question and no reason is given as to why the intervening steps or gradations will simply be bypassed.
This "argument" has the following form:
This sort of "reasoning" is fallacious because there is no reason to believe that one event must inevitably follow from another without an argument for such a claim. This is especially clear in cases in which there is a significant number of steps or gradations between one event and another.
"We have to stop the tuition increase! The next thing you know, they'll be charging $40,000 a semester!"
"The US shouldn't get involved militarily in other countries. Once the government sends in a few troops, it will then send in thousands to die."
"You can never give anyone a break. If you do, they'll walk all over you."
"We've got to stop them from banning pornography. Once they start banning one form of literature, they will never stop. Next thing you know, they will be burning all the books!"
If we pass laws against fully-automatic weapons, then it won't be long before we pass laws on all weapons, and then we will begin to restrict other rights, and finally we will end up living in a communist state. Thus, we should not ban fully-automatic weapons.
You should never gamble. Once you start gambling you find it hard to stop. Soon you are spending all your money on gambling, and eventually you will turn to crime to support your earnings.
If I make an exception for you then I have to make an exception for everyone.
Use the "if-then" operator with caution and make sure the relationship is supported by evidence.
If your opponent uses a slippery slope argument, show that the final event need not occur as a consequence of the original proposition.