Similarity is a notion that is widely used both in cognitive science and in argumentation theory. These research programs have, however, developed in large part separately and in consequence rely on disparate notions of similarity. Only recently there has been a proposal for specifying how similarity actually plays a role in judging slippery slope arguments. We present here further theoretical discussion and empirical evidence in order to show how similarity can play a role in slippery slope arguments and in argumentation in general. In the experiment presented here, we manipulated the availability of causal information, and showed that people are sensitive to it when judging arguments’ strength. We conclude that similarity between causal properties of the elements presented in arguments is crucial for arguments’ strength assessments.