To what extent do moral judgments depend on conscious reasoning from explicitly understood principles? We address this question by investigating one particular moral principle, the principle of the double effect. Using web-based technology, we collected a large data set on individuals' responses to a series of moral dilemmas, asking when harm to innocent others is permissible. Each moral dilemma presented a choice between action and inaction, both resulting in lives saved and lives lost. Results showed that: (1) patterns of moral judgments were consistent with the principle of double effect and showed little variation across differences in gender, age, educational level, ethnicity, religion or national affiliation (within the limited range of our sample population) and (2) a majority of subjects failed to provide justifications that could account for their judgments. These results indicate that the principle of the double effect may be operative in our moral judgments but not open to conscious introspection. We discuss these results in light of current psychological theories of moral cognition, emphasizing the need to consider the unconscious appraisal system that mentally represents the causal and intentional properties of human action.