Slippery slope arguments (SSAs) have a bad philosophical reputation. They seem, however, to be widely used and frequently accepted in many legal, political, and ethical contexts. Hahn and Oaksford (2007) argued that distinguishing strong and weak SSAs may have a rational basis in Bayesian decision theory. In this paper three experiments investigated the mechanism of the slippery slope showing that they may have an objective basis in category boundary re-appraisal. When the beginning and the end of a slippery slope are more similar, the probability that they are perceived to belong in the same category is higher and the SSA is stronger. Experiment 1 established a robust effect of probability on SSA evaluation. Experiments 2 and 2A showed that when similar items are classified in the same category this leads to stronger SSAs. In Experiment 3, in a correlational analysis, it was shown that participants' confidence in their categorisation judgements predicted the perceived strength of an SSA and that this relationship was moderated by similarity between the ends of the slippery slope. We conclude that an important aspect of many SSAs may have an objective basis in well-established and rational cognitive theories. (Contains 6 figures.)