Slippery slope. Look for a pattern where one asserts that if a position is accepted, then it will lead to something that's not good, and that in turn will lead to something worse, until we arrive at a place so terrible that everyone would agree that it would be bad. And on that basis, it is said that we should not accept the first position. This pattern of SLLLLIPPPING down down down is what makes this fallacy easy to spot.

Explanation:

In order to show that a proposition P is unacceptable, a sequence of increasingly unacceptable events is shown to follow from P. A slippery slope is an illegitimate use of the IF-THEN operator.

Examples:

(i) 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.

(ii) 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.

(iii) If I make an exception for you then I have to make an exception for everyone.

Proof:

Identify the proposition P being refuted and identify the final event in the series of events. Then show that this final event need not occur as a consequence of P.

Important: The following types of slippery slopes are not usually fallacious.

1. Observation of a slippery slope in history.

For example, all forms of entertainment media have gradually lowered their standards of what should be portrayed. This includes the increasing courseness of jokes, increased violence, increased nudity, etc. It is not a fallacy to point this out. It is just a simple fact.

2. Observing a historical slippery slope, then using it as the basis for an argument by analogy.

For example, Leonard Peikoff demonstrates the parallel philosophical tracks of pre-Nazi Germany and contemporary America. For instance, Germany gradually devalued human life. America continues to devalue human life. From this (and many other parallels) he concludes that the United States is in danger of becoming like Nazi Germany.

3. Higher Induction: Certain inductive conclusions can be drawn based on understanding of human nature.

For example, it is well understood that when people begin certain behaviors or practices, the behavior increases in intensity over time. This is the nature of addiction. So it is reasonable to warn people about the dangers of taking illegal drugs. It is highly likely that they will get more and more seriously involved with them.