bilateral contracts, and other classifications of contracts in Roman law, are in part superficial groupings based on a single and often external aspect such as the kind of formalities they are associated with; and in part they confuse (among other things) determinations which concern the nature of contract itself with others which concern only the administration of justice (aaiOtzes) and the legal rechtliclzen consequences of positive law, and which often derive from wholly external circumstances and contravene the concept of right.