In December 1996 Classification of Obligations formed the topic of one of a series of SPTL seminars under the general title of Pressing Problems in the Law. It may, perhaps, be asked quite why classification is a pressing problem, for it is by no means clear from the papers themselves that common lawyers have suddenly become more concerned about the internal structure of the 'seamless web'. Nevertheless the seminar was a valuable opportunity to reflect upon a subject that is at least a useftil vehicle for thinking about legal knowledge. Legal classification, in other words, raises questions of an epistemological nature. The purpose of this present paper is to pursue this epistemological point in an attempt to reveal how classification of symbolic knowledge (legal propositions or rules) hides much deeper issues about the role of non-symbolic knowledge (symmetries, images and isomorphs) in the formulation of legal solutions in the law of obligations. In pursuing this aim the article will draw on and comment upon, if only briefly, a range of papers submitted to the seminar. In addition it will try to set out the background both to legal classification and to some recent developments in aspects of epistemological (and in particular systems) thinking. And it will try to do this in the context of a comparative exercise in the hope of reinforcing the point that the time has come for comparative law (incorporating Roman law), as a subject, to assume a more theoretical - that is to say, epistemological - role in legal studies and legal education.