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The slippery slope is an argument from the effect of the occurrence of one event on the likelihood of the occurrence of another. The debater who employs the slippery slope maintains that event A should be avoided because it will increase the probability of—if not make inevitable—the occurrence of undesirable event B. In the realm of political debate, the slippery slope is very frequently fallacious.
The argument is not in itself a fallacy because there are situations in which event A should be avoided because the detrimental event B necessarily follows A. As a simple example, a reasonable person naturally understands that punching a wall should be avoided because this event leads via a chain of subsequent events to the end result of the sensation of pain. Reasoning by slippery slope that punching a wall is undesirable is indeed valid.
The flaw in the slippery slope as a logical flow is that it involves a specific understanding of the nature of causation in any given scenario. Firstly, the connection between events A and B must be verified for the slippery slope to carry weight; the argument’s strength is only as great as the certainty and degree of the connection. Secondly, the implications of a particular probability are arbitrary and, moreover, usually ill-defined. The third flaw is the most pertinent in relation to the political arena: when both events A and B involve decision, the avoidance of choice A springs from the condemnation not of choice B per se but of the deterrence of being in a situation in which making choice B is more likely. The situation in which events A and B are both choices involving reasonable decision-making and the situation in which event A is a decision and event B is, say, a natural reaction differ subtly in the presence of the faculties of reason in both events in the former case.
The specificity of this situation belies its perfunctory role in politics. Frequently one party makes the argument that policy A should not be enacted because policy B is likely to follow. In this example the third flaw of the slippery slope invalidates the legitimacy of the logical flow of the argument because the enactment of both policies entails the reasoning process. Policy B is no more likely to pass regardless of the passage of policy A, assuming the decision is based on sound logic. This assumption is trivial because the debate over policy A presupposes the usage of reason (otherwise the validity of the slippery slope would be irrelevant). Rarely do political decisions reflect the reasoning process, and so policy B may indeed be more likely due to policy A because the latter skews public perception of the succeeding policies. The crux of the slippery slope in these political scenarios, thus, is the degree to which policy A makes policy B more likely to pass.
As long as the reasoning process is present in the decision, however, the passage of policy A should have no logical bearing upon the debate over policy B. Logic deals with ideal decisions and so the disturbance in the employment of this tool in legislating policy B is logically (although perhaps not politically) irrelevant.