The developments in 12th- and 13th-century legal procedure, known as inquisitio, illustrate that there existed a learned and reasoned legal system used by both secular and ecclesiastical courts in continental Europe. This essay outlines how scholarship has re-defined and provided greater understanding of the paradox between the granting and denying of due process. The interpretation that inquisitorial procedure itself afforded rights to the defendant in the court of law is set against the suspension of those rights for the perceived threat of heresy. Two important historiographical approaches offer a politically motivated and intellectually grounded explanation for this deviation from or adherence to procedural norms: one views inquisitorial procedure within the context of the Church's growing power to legislate and the other views procedure within the context of ideas about natural law that guaranteed a defendant's rights. The juxtaposition of natural law safeguarding procedural rights with the duty to protect society from the threat of heresy, thereby justifying deviations from procedure, resonates in our post-9/11 world. Today's terrorism is yesterday's heresy. Be it the 13th century or the 21st century, the balance between procedure and the maintenance of due process is problematic.