Helge Dedek, McGill University Faculty of Law, and Martin Schermaier have published Obligation (Greek and Roman), in the Encyclopedia of Ancient History (Roger Bagnall, Kai Brodersen, Craige Champion, Andrew Erskine, and Sabine Huebner eds., Oxford: Wiley & Blackwell, 2011). Here is the abstract.
Justinian’s Institutes defined the concept of obligation as follows: Obligatio est iuris vinculum quo necessitate adstringimur alicuius solvendae rei secundum nostrae civitatis iura (Iustinian Institutiones 3.13.pr): The obligatio is a "legal tie" (iuris vinculum) that binds us to render a performance to another person according to our laws. This definition timelessly expresses the nature of an obligation: a debtor owes a duty to the creditor. The content of such a duty, and exactly how it may or must be performed, are infinitely variable and determined by the event that gives rise to the obligation, not by the concept of obligation itself. When we examine the idea of an obligation, it is possible to distinguish between the debtor’s duty and the debtor’s potential liability. The concept of "duty" expresses that someone (the debtor) owes something to another (the creditor). The concept of "liability" adds that the debtor can be held responsible if he breaches such a legal duty: the creditor can seek the assistance of the courts if he does not receive what was owed to him.Download the text from SSRN at the link.