Internet-based tools that encourage public participation in debates concerning policy issues have been recognized as a good way to engage the electorate with political issues. In addition, such systems for e-participation can gather, make available, and analyze the public's contributions to political debate. In this article we discuss a system called Parmenides, which we designed to exploit technological developments to bring democratic processes into the online world. Parmenides is primarily a forum by which government bodies can present policy proposals to the public so that users can submit their opinions on the justification presented for the particular policy. Within Parmenides, the justification for action is structured to exploit a specific representation of persuasive argument based on the use of argumentation schemes and critical questions.