If Christ exists in two natures, as the Council of Chalcedon proclaims, does it follow that he performed distinct classes of activities, human and divine? If you say yes, you are a “dyoenergist” (from the Greek dyo energeiai, “two activities”); if you insist that Christ performed activities of only one class, and thus answer in the negative, you are a “monoenergist” (from the Greek for “one activity”). The dyoenergist position was defended in memorable fashion in the Synodical Letter of Sophronius of Jerusalem from the year 634. To understand this letter, as well as several documents that follow in this volume, we must set the historical context and map some difficult conceptual terrain.