Causal knowledge can be based on acquired information about the statistical relationship (covariation) between a cause and effect or on knowledge of the mechanism by which causal power is transmitted between the cause and effect. A key issue is the functional significance of this distinction. In this article, we review recent research in which the influence of covariational evidence on prior beliefs was analyzed. We argue that the way in which covariation influences prior beliefs is independent of whether those beliefs are based on covariation or mechanism information, and that convincing demonstrations of the dissociability of the two types of causal knowledge have not been obtained. We argue that although there are several ways in which causal knowledge can be acquired, that knowledge shares a common representational basis.