Following an idea first proposed by Jerzy Wróblewski, this paperexamines the usefulness of formal logic for comparative legal analysis. Subject of the comparison are the doctrines of mistake and attempt in Germanand English criminal law. These doctrines are distinguished by the interaction of deontic, epistemic and alethic modalities. I propose a purely extensional logic which is based on Leśniewski’s substitutional interpretation ofquantification to analyse differences in the logical structure of the variouscriminal law doctrines.