A slippery slope argument (SSA), in logic, critical thinking, political rhetoric, and caselaw, is a consequentialist logical device in which a party asserts that a relatively small first step leads to a chain of related events culminating in some significant (usually negative) outcome.
The core of the slippery slope argument is that a specific decision under debate is likely to result in unintended consequences. The strength of such an argument depends on the warrant, i.e. whether or not one can demonstrate a process that leads to the significant effect. This type of argument is sometimes used as a form of fear mongering, in which the probable consequences of a given action are exaggerated in an attempt to scare the audience.
The fallacious sense of ‘slippery slope’ is often used synonymously with ‘continuum fallacy,’ in that it ignores the possibility of middle ground and assumes a discrete transition from category A to category B. In a non-fallacious sense, including use as a legal principle, a middle-ground possibility is acknowledged, and reasoning is provided for the likelihood of the predicted outcome.
Other idioms for the slippery slope argument are the ‘thin end,’ ‘edge of the wedge,’ ‘the camel’s nose in the tent,’ or ‘If You Give a Mouse a Cookie.’ A related concept in criminology is the ‘broken windows theory,’ which argues that failure to police minor crimes leads to an increase in more serious crimes.
A slippery slope argument is typically a negative argument where there is an attempt to dissuade someone from taking a course of action because if they do it will lead to some unacceptable conclusion. Some writers point out that an argument with the same structure might be used in a positive way in which someone is encouraged to take the first step because it leads to a desirable conclusion.
There are two commonly distinguished types of slippery slope arguments. In ‘causal’ slopes, each step in the chain of events is the cause of the next in the sequence. ‘Judgmental’ or ‘decisional’ slopes consists of a series of judgement where each inevitably requires another other decision to be made until there is a final outcome.
The judgmental type has a further subdivision called the conceptual’ slippery slope, which Canadian philosopher Trudy Govier calls the fallacy of ‘slippery assimilation. The concept refers to a categorization problem similar to the ‘sorites paradox,’ sometimes known as the ‘paradox of the heap,’ which asks what is the minimum number of grains of sand that would comprise a ‘heap.’
Argumentation theorist Christopher Tindale called slippery slope reasoning ‘a type of negative reasoning from consequences, distinguished by the presence of a causal chain leading from the proposed action to the negative outcome.’ Logician Doug Walton says that an essential feature of decisional slippery slopes is a ‘loss of control.’
Philosopher T. Edward Damer, in his book ‘Attacking Faulty Reasoning,’ describes what others might call a causal slippery slope but prefers to call it the ‘domino fallacy.’ Logician Howard Kahane suggests that the domino variation of the fallacy has gone out of fashion because it was tied the ‘domino theory,’ which was used as a justification for U.S. involvement in the Vietnam War. It posited that if one country in a region came under the influence of communism, then the surrounding countries would follow.
German criminologist Frank Saliger notes that ‘in the German-speaking world the dramatic image of the dam burst seems to predominate, in English speaking circles talk is more of the slippery slope argument.’ In exploring the differences between the two metaphors he comments that in the dam burst the initial action is clearly in the foreground and there is a rapid movement towards the resulting events, whereas in the slippery slope metaphor the downward slide has at least equal prominence to the initial action and it ‘conveys the impression of a slower ‘step-by-step’ process where the decision maker as participant slides inexorably downwards under the weight of its own successive (erroneous) decisions.’
Lawyer Eric Lode notes that ‘commentators have used numerous different metaphors to refer to arguments that have this rough form. For example, people have called such arguments ‘wedge’ or ‘thin edge of the wedge,’ ‘camel’s nose’ or ‘camel’s nose in the tent,’ ‘parade of horrors’ or ‘parade of horribles,’ ‘domino effect,’ ‘boiling Frog,’ and ‘this could snowball’ arguments. All of these metaphors suggest that allowing one practice or policy could lead us to allow a series of other practices or policies.’
The Daily Omnivore 
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