The slippery slope argument appears often in political, theological, and ethical debates. Simply put, it argues that accepting one questionable idea leads to a downward slide to worse and worse ideas and finally to an abyss of absurdity. It burdens the first idea with the weight of the last. Political progressives use it. Political conservatives use it. Theological liberals and theological traditionalists love it. And all parties criticize each other for making it! When it supports our side we think it brilliant and when it doesn’t we reject it as fallacious. Which is it?
Both the admiring and dismissive reactions to the slippery slope argument (SSA) are evoked by the claim that the cascade of ideas moving from bad to worse to absurd are linked by necessary relationships so that accepting the first leads inexorably to the last. The paradoxical tendency of the argument to provoke both reactions lies in its confusing combination of very different kinds of relationships used to map the descending cascade: logical, psychological, sociological, and individual tastes and preferences. An SSA is strongest when it relies heavily on logical relationships. Such an argument begins with clear definitions and principles, true premises, and progresses with valid inferences to its conclusions. In politics, theology, and morality purely logical arguments are too abstract to get the job done. In these areas where so much more than truth is at stake—money, pleasure, power, and honor—rarely can people on different sides of a debate agree even on definitions, principles, and foundational premises; and without clarity and agreement on a common beginning point, each move’s validity will be called into question.
Confusion is compounded when psychological and sociological connections are presented as if they were logical inferences. Our understanding of human psychology and particular behavior patterns associated with particular psychological states is derived from empirical experience. Human behavior patterns derived from empirical experience cannot infallibly be extrapolated into future behavior. I doubt this would be possible even if we assumed that the exact same conditions will obtain for a future act as were present for the observed past behavior. But of course, conditions are never the same, and there are way too many factors to take into account, many of them hidden. Moreover, human beings are highly susceptible to influence from the concentric circles of groups to which they belong. Behavior that would not make psychological sense when acting alone makes perfect sense when contemplated in its sociological dimension and vice versa. Lastly, individual human beings differ from each other in ways that are unpredictable from the usual psychological and sociological patterns.
Hence the SSA must be used with caution and evaluated with a critical eye. And yet, it is incorrect to label its every use a “fallacy.” Even at the psychological and sociological levels human behavior falls into repeating patterns that can be somewhat predictive. Also, even though not all human behavior patterns can be described in purely logical terms and people are not logical machines, our minds are structured in a way that we experience dissonance when we are confronted with a tension between our desire for money, pleasure, power, and honor on the one hand and truth, fact, and logical coherence on the other. Hence behaving rationally, which includes tracing out and accepting the implications of one’s basic axioms, is a psychological need as well as a rational duty. Social pressure, too, can drive one to seek praise from others for being courageous enough to take the next bold step in unfolding the logic of one’s foundational premise.
Hence the SSA can be a sound and persuasive argument if proper attention is paid to its different dimensions: logic, psychology, sociology, and individual variability. It cannot show us what will happen if we adopt a particular axiom as foundational, but it can show us what might happen if we do so. And that may be enough to provoke some to engage in serious reflection before they embark on a journey whose downward trajectory leads to the abyss of absurdity.
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