Explanations of many cognitive processes, including probability judgment, rely on the construct of similarity. The present paper is concerned with the similarity-based explanation of reasoning in the conjunction task. Although high positive correlations have been found between similarity and probability judgments in this task, these alone cannot validate the assumption that similarity is judged by a process of feature comparison or that similarity judgment is an explanation of probability judgment. Preliminary results from a study in which we collected written justifications from subjects who made both types of judgment suggest that these assumptions are not tenable. Subjects cited considerations of causality and statistics ~ not just feature overlap -- when judging both similarity and probability, indicating that (1) feature comparison is only one way in which people judge similarity and (2) similarity judgment can involve processes usually associated with probability judgment. These findings suggest that the role of similarity in explaining other cognitive processes needs to be revised. It is proposed that the power of similarity and probability to predict one another can be exploited for the purpose of making either type of judgment.