Logic

Is a "slippery slope" argument the same as a reductio ad absurdum?

Comments

No, they're not the same. A "slippery slope" argument is one that tries to show that attempting to make a determination about one sort of case will "slide" into another case very like it, and then another very like that one, and so on, until we reach a point where we are no longer willing to endorse the original strategy. For example, a familiar version of "slippery slope" reasoning holds that if we grant rights to gays, then the next thing that will happen is that we have to grant rights to pedophiles, or those who enjoy bestiality or incest. The inference we are suppose to make is that wwe should not take the first step down the slope, as there will be no stopping all of the steps that follow "once you go down that path." As this example shows, many "slippery slope" arguments are simply fatuous, and as a matter of fact none are actualluy logically valid.

A reductio ad absurdum, however, is a logically valid form of argument. The way this argument form works is in virtue of a certain fact about conditional statements (if p then q). The truth conditions of these statements are such that all instances of this form will be true except when the value of the antecedent (p) is true and the value of the consequent (q) is false. Given this fact about conditionals, if you can prove that a statement (say, r) entails some known falsehood (say, s), then you have provided a logical ground for counting r as false. The most obvious case of this will be one in which a statement (say, t) entails a contradiction (both u and not-u).

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