Claiming that a modest first step will inevitably lead to an extreme final outcome — through a chain of events that is presented as automatic but is rarely argued.
"If we allow the government to mandate vaccines for schoolchildren, what's next? Mandatory medication for everyone? Complete control over our bodies? This is how totalitarianism starts." Each step in the chain is treated as automatic. But the progression from childhood vaccine requirements (a policy with a century of precedent) to totalitarianism involves dozens of contested political, legal, and institutional steps.
We are psychologically wired to fear worst-case scenarios, and chains of causation feel plausible when we don't examine each link. The slippery slope also exploits the fact that most catastrophes do have gradual origins — which makes the pattern feel predictive when it's actually unfalsifiable.
Arguments that skip from policy X to catastrophic outcome Z without defending each step. Ask: "What are the intermediate steps, and what prevents them from happening?"