The term slippery slope refers both to an argument about the likelihood of one event given another, and to a fallacy about the inevitability of one event given another. It is also known as the thin end of the wedge or the camel's nose.

Argument

As an argument, it takes the form

If A occurs

B is more likely to occur

The argument is that by making a move in a particular direction, we are starting down a "slippery slope" in which it is likely that we will continue in the same direction (usually deemed by the arguer to be a negative one; hence the "sliding downwards" metaphor). One example is the argument by many civil libertarians that even minor increases in government authority make future increases more likely, by making them seem less noteworthy: what would once have been considered a huge power grab, the argument goes, is now seen as just another incremental increase, and thus is more palatable (see, e.g. [1]).

Eugene Volokh's Mechanisms of the Slippery Slope (PDF) analyzes various types of such slippage. Volokh uses the example "gun registration may lead to gun confiscation" to describe five types of slippage:

• Cost-lowering: Once all guns are registered, the government will know exactly who to confiscate them from.
• Legal rule combination: Previously the government might need to search every house to confiscate guns, and such a search would violate the Fourth Amendment of the Constitution of the United States. Registration would eliminate that problem.
• Attitude altering: People may begin to think of gun ownership as a privilege not a right, and thus think of gun confiscation not as seriously.
• Small change tolerance: People may ignore gun confiscation because it's a small change, but when combined with other small changes, it could lead to the equivalent of registration.
• Political power: The hassle of registration may reduce the number of gun owners, and thus the political power of the gun ownership bloc.
• Political momentum: Once this gun law is passed it becomes easier to pass other gun laws, including laws like confiscation.

Fallacy

As a logical fallacy the argument takes the form

1. A has occurred (or will or might occur).
2. Therefore B will inevitably happen.

Where B generally dwarfs A in significance. The "camel's nose" example is: once a camel has managed to place its nose within a tent, the rest of the camel will inevitably follow.

The fallacy is that such a claim requires an argument connecting the inevitability of B to A.

The slippery slope fallacy is often connected to the straw man fallacy to attack the initial position:

1. A has occurred (or will or might occur).
2. Therefore B will inevitably happen. (slippery slope)
3. B is wrong; therefore A is wrong. (straw man)

This form of argument is often used as a commentary on social change. The argument holds that once an exception is made to some rule, there will be nothing holding back further, more egregious exceptions to that rule.

Contemporary examples of the slippery slope fallacy in use:

• If we allow women to abort their unborn children, then soon no life will be held sacred.
• If we allow guns to be registered, then gun confiscation will follow.
• Use of 'soft' drugs such as cannabis will inevitably lead to addiction to 'harder' drugs such as heroin.

These arguments depend on a perception of momentum in the change of social mores.