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In logic and critical thinking, a slippery slope is a logical device, but it is usually known under its fallacious form, in which a person asserts that some event must inevitably follow from another without any rational argument or demonstrable mechanism for the inevitability of the event in question. A slippery slope argument states that a relatively small first step leads to a chain of related events culminating in some significant effect, much like an object given a small push over the edge of a slope sliding all the way to the bottom. 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. 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. Modern usage avoids the fallacy by acknowledging the possibility of this middle ground.
The argument takes on one of various semantical forms:
The core of the slippery slope argument is that a specific rule or course of action is likely to result in unintended consequences and that these "unintended consequences" are undesirable (and, typically, worse than either inaction or another course of remediation). This criticism is a consequentialist criticism - interested in consequences or outcomes or results of a course of action - and does not impugn the character or intentions of the one(s) offering the "slippery slope" argument(s), the basis or bases or concerns underlying the offering of the arguments against a rule or course of action, nor the legitimacy of arguing against any specific rule or course of action.
Eugene Volokh's Mechanisms of the Slippery Slope (PDF version) analyzes various types of such slippage. Volokh uses the example "gun registration may lead to gun confiscation" to describe six types of slippage:
Slippery slope can also be used as a retort to the establishment of arbitrary boundaries or limitations. For example, someone who is unfamiliar with the possible negative consequences of price ceilings might argue that rent prices must be kept to $1,000 or less a month to be affordable to tenants in an area of a city. A retort invoking the slippery slope could go in two different directions:
Sometimes a single action does indeed induce similar later action. For example, judiciary decisions may set legal precedents.
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Several common analogies support slippery slope arguments. Among these are analogies to physical momentum, to frictional forces and to mathematical induction.
In the momentum analogy, the occurrence of event A will initiate a process which will lead inevitably to occurrence of event B. The process may involve causal relationships between intermediate events, but in any case the slippery slope schema depends for its soundness on the validity of some analogue for the physical principle of momentum. This may take the form of a domino theory or contagion formulation. The domino theory principle may indeed explain why a chain of dominoes collapses, but an independent argument is necessary to explain why a similar principle would hold in other circumstances.
An analogy similar to the momentum analogy is based on friction. In physics, the static co-efficient of friction is always greater than the kinetic co-efficient, meaning that it takes more force to make an object start sliding than to keep it sliding. Arguments that use this analogy assume that people's habits or inhibitions act in the same way. If a particular rule A is considered inviolable, some force akin to static friction is regarded as maintaining the status quo, preventing movement in the direction of abrogating A. If, on the other hand, an exception is made to A, the countervailing resistive force is akin to the weaker kinetic frictional force. Validity of this analogy requires an argument showing that the initial changes actually make further change in the direction of abrogating A easier.
Another analogy resembles yet misinterprets mathematical induction. Consider the context of evaluating each one of a class of events A1, A2, A3,..., An (for example, is the occurrence of the event harmful or not?). We assume that for each k, the event Ak is similar to Ak+1, so that Ak has the same evaluation as Ak+1.
Therefore every Ak has the same evaluation as A1.
For example, the following arguments fit the slippery slope scheme with the inductive interpretation:
This argument instantiates the slippery slope scheme as follows: Ak is the situation in which k building permits are issued. One first argues that the situation of k permits is not significantly different from the one with k + 1 permits. Moreover, issuing permits to build 1000 religious structures in a city of 300,000 will clearly change the nature of the community.
In most real-world applications such as the one above, the naïve inductive analogy is flawed because each building permit will not be evaluated the same way (for example, the more religious structures in a community, the less likely a permit will be granted for another).