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Slippery Slope

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. This fallacy, a subgenre of the appeal to consequence, is also known as "The camel’s nose". "In a typical slippery slope argument, an action is objected to on the grounds that once it is taken, another, and then perhaps still another, are bound to be taken, down a ‘slippery slope,’ until some undesirable consequence results. According to a slightly different version, whatever would justify taking the first step would also justify all the others, but since the last step isn’t justified, the first isn’t, either." (Kahane & Cavender, Logic and Contemporary Rhetoric, p. 84)

Examples
  1. "If abortion on demand were to become legal, there would be a great increase in abortions. And once abortion became commonplace, there would be a weakening of respect for human life in general. Once the respect for human life was weakened, we would see an increase in euthanasia of all kinds: the elderly, the mentally handicapped, and the physically disabled. Before long we would be getting rid of anyone who is unproductive. In short, it would threaten our civilization. Therefore, we should oppose any move to broaden the grounds for legal abortions." (Hughes & Lavery, Critical Thinking, pp. 167-8.) 
  2. "One of the arguments against court-ordered desegregation of the schools was that if we allow the court to determine which public schools our children will attend, the court will also tell us whom we have to allow into our churches, whom we have to invite into our homes, and even whom we should marry. In this example, the action (court-ordered desegregation) lies on a continuum with the court ordering whom we should marry at an extreme end. The argument being made is that if we allow the court to have jurisdiction over events at one end of the continuum, then it will take over the other events on the continuum." (Halpern, Critical Thinking Across the Curriculum, p. 128.)
  3. "The more disaster-prone prevention education contains the 'logical' statement that any form of drinking (especially by young people) will lead to more drinking and will eventually result in death. Aside from its inaccuracy, most students who hear this warning are not impressed and may scoff at the whole notion. Mot people in the world drink and the drinking does not increase pathologically or end in premature death. In fact, some data indicate that a drink or two is good for you." (Taleff, Critical Thinking for Addiction Professionals, p. 108.)
  4. If I make an exception for you then I have to make an exception for everyone.
  5. "Whether a causal type of domino effect is reasonable depends on the strength or plausability of the evidence given to support the causal linkages proposed at each step. The classic case of the domino effect argument was its use during the Vietnam War era to argue that, if Vietnam fell to the Communists, then neighboring countries like Cambodia would also fall. Then, other adjacent countries would fall until the whole of East Asia would be in Communist hands. This argument was often used as a kind of scare tactic by its exponents, and because not much evidence seemed available to back it up very firmly, it came to be thought of as a fallacious type of argument, in this particular instance." (Walton, Informal Logic, pp. 268-9.)
Critique

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.

Comment

Action and Consequence

Some consequences can be foreseen with a high degree of assurance. While our foreknowledge cannot be perfect, life would be impossible if we could not regularly anticipate the effects of our actions. What is required for a slippery slope argument to stand up to scrutiny, then, is that the inevitability (or at least likelihood) of foreseen consequences be sufficiently demonstrated.

"Note, however, that some slopes may well be slippery. The slippery slope fallacy is committed only when we accept without further justification or argument that once the first step is taken, the others are going to follow, or that whatever would justify the first step would in fact justify the rest. Note, also, that what some see as the undesirable consequence lurking at the bottom of the slope others may regard as very desirable indeed..." (Kahane & Cavender, Logic and Contemporary Rhetoric, p. 85)

Bowell and Kemp agree...

"This fallacy occurs when an arguer wrongly assumes that to permit or forbid a course of action will inevitably lead to the occurrence of further related and undesirable events, without providing good reasons to suppose that the further events will indeed inevitably follow; and thus to allow the first is to tread on a slippery slope down which we will slide to the other events" (Bowell & Kemp, Critical Thinking, p. 157.)

Of course, considering the most likely consequences of our actions is not only unavoidable, it is a moral responsibility that is essential to living wisely. Such deliberations, though, should include a careful and honest evaluation of the probabilities, and not be merely scare tactics devised for the sake of winning an argument. This responsibility is all the more urgent for policymakers, whose decisions stand to effect whole populations.

We often want to assess a proposal to take some action or adopt a new policy. One important and legitimate way of doing so is to examine the consequences that would result if the action were taken or the policy adopted. If the action or policy is likely to lead to undesirable consequences, the we have a good reason to reject it, and if it is likely to lead to desirable consequences, then we have a good reason to support it. This method of assessment involves making prediction about the future, and it is important to realize that such predictions are always somewhat tentative. Even when there is a wealth of empirical support for prediction we know how easily they can be wrong: think of weather forecasts and stock market predictions. (Hughes & Lavery, Critical Thinking, pp. 166-7.)

On the difficulty of anticipating consequences and isolating cause and effect...

"There are two distinct problems with this relocation of the debate over euthanasia and physician-assisted suicide on slippery-slope turf. For one thing, as we have seen, slippery-slope arguments involve predictive empirical issues about possible future abuse, and the evidence for such claims cannot be firm; however, slippery-slope arguments cannot be disconfirmed either, at least not in the weak sense that undesirable consequences B could always follow A, whether or not they were actually caused by A or caused by some other force in the presence of A as a precedent. Slippery-slope arguments are always around; they rarely dissipate until long after the social change in question has already taken place. However, because of their power to persuade, especially in a broad, public context, slippery-slope arguments tend to block out other major concerns that should be regarded as central too, or perhaps as still more central. Thus, on both counts, the structure and location of the debate skew the answer." (Battin, Ending Life, p. 34-5.)

By that same logic...

The second type of slippery slope to which Hughes and Lavery refer in our summary is worth special consideration: "According to a slightly different version, whatever would justify taking the first step would also justify all the others, but since the last step isn't justified, the first isn't, either." Let's consider a very controversial example. Those who argue against government sanctioned gay marriage are often accused of proposing a slippery slope because they regularly suggest that by the same logic we will have to sanction polygamous marriage, adult/child marriage, and even the man who wants to marry his horse (which apparently has happened a number of times without government sanction). These are not necessarily seen as likely political consequences, but rather the defender of traditional marriage is making a point about the logic of sanctioning gay marriage. As they see it, it is not merely a matter of redefining marriage, but rather of conceding that marriage has no intrinsic definition whatsoever. If the definition of marriage is not determined on the basis of tradition, religion, or natural law; if it is merely a malleable social construct; then by what logic can anyone who desires to be married, for whatever reason, be deprived of government sanction? Whatever one thinks of the traditionalist's argument in this case, slippery slope arguments of this sort can serve to clarify the logic of the argument that is being made for the first step onto the slope.