A slippery slope fallacy is a form of reasoning where it is argued that taking one step in some direction will inevitably (or with very high probability) result in a number of subsequent steps being taken in the same direction, culminating in some extremely undesirable outcome. Slippery slope arguments are rhetorically powerful because they can discredit a seemingly innocuous action by linking it with a clearly undesirable, extreme outcome that it is argued to be likely to bring about. As for most informal fallacies, slippery slope reasoning is not always fallacious. Whether or not a given instance of slippery slope reasoning is fallacious depends upon how carefully the inevitability of the steps linking the initial to the final undesirable outcome has been established. This is sometimes phrased in turns of how compelling is the argument that the slope is truly ‘slippery’. A slippery slope fallacy is only committed when it is asserted that a slope is slippery without sufficient argument or justification. Consider first an example of a valid slippery slope argument:
“Be careful with that ball near the stairs. If the ball falls down onto the first step, it will bounce around and fall down to the second step, and from there to the third step, and so on until eventually it will fall all the way down to the bottom of the steps, and then you’ll have to go all the way down to pick it up.”
This argument gives a specific reason as to why the ball initially falling down the first step (which is presumably in itself not a problem) will lead to it falling down onto the second step, and so on eventually until it reaches the bottom (presumably a highly undesirable outcome). As such, the argument has clearly established the ‘slipperiness’ of the slope, and so no fallacy is committed. Consider now an example of an invalid (fallacious) slippery slope argument:
“If we introduce restrictions on freedom of speech, before long pressure groups will by lobbying politicians for more and more restrictions, and eventually we will end up in a police state where everything we say must be approved by the government.”
This argument is fallacious because it is not made clear why introducing some restrictions on freedom of speech would inevitably, or even likely, lead to more and more restrictions, culminating in a police state. Although it is possible that introducing some restrictions on freedom of speech could lead to more being introduced, no reason is given as to why this would necessarily occur, or why this would continue until the very extreme outcome of a police state was reached. Thus, this argument fails to establish that the slope is truly as ‘slippery’ as has been implied. Perhaps with some additional arguments, the slippery slope fallacy could be avoided, but as it stands this argument merely asserts the extreme outcome without providing adequate basis for it, and therefore commits the slippery slope fallacy.
The slippery slope argument is most common in legal and moral debates, where it is common to hear the claim that if such and such is allowed, or if some policy is enacted, then it will inevitably lead to some extreme outcome of allowing horrific crimes, or of terrible policies that none would support. Unless very specific, clear reasons are given as to why the first step in the process would necessarily lead to the extreme case, such inferences commit the slippery slope fallacy.
Slippery slope fallacy: overview from the Nikor Project
Slippery slope fallacy: overview from Logical Fallacies
Slippery slope: overview from Logically Fallacious
The slippery slope: useful introduction with examples from The Logical Place