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:
In the classical form, the arguer suggests that making a move in a particular direction starts something on a path down a "slippery slope". Having started down the metaphorical slope, it will continue to slide in the same direction (the arguer usually sees the direction as a negative direction).
Modern usage includes a logically valid form, in which a minor action causes a significant impact through a long chain of logical relationships. Note that establishing this chain of logical implication (or quantifying the relevant probabilities) makes this form logically valid. The slippery slope argument remains a fallacy if such a chain is not established.[dubious– discuss]
Consequentialism and unintended consequences
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.