fallacy

Slippery slope

Also called: Domino fallacy, Thin end of the wedge

A slippery slope argument claims that one step will set off a chain of events ending in disaster, without showing that each link in the chain is likely.

How it works

The argument borrows the emotional weight of the final outcome and transfers it to the first, modest step. Each link may sound plausible on its own, but the probability of the whole chain is much lower than any single step.

Examples

"If we let students redo one exam, soon they'll expect to redo every exam, grades will mean nothing, and universities will collapse."

No evidence is offered that a single resit policy leads to each following step.

"Allow one food truck in the square today, and next year the whole town centre will be a car park for vans."

The leap from one licence to the loss of the town centre is asserted, not shown.

How to spot it

A chain of "then... then... then..." ending in catastrophe.
No evidence for the individual links.
The first step is reasonable on its own terms.

How to respond

Ask which link is the weakest and what evidence supports it.
Point out where a line can be drawn and enforced.
Accept that some slopes are real when there is evidence of momentum, and ask for that evidence.