  Other writers have attempted to supply a more formal definition of, for example, a minimally coherent legal system (see Levenbook 1984), or otherwise to flesh out in a more detailed manner the criteria of coherence. Alexy and Peczenik 1990 define coherence in terms of the degree of approximation to a perfect supportive structure exhibited by a set of propositions, and list ten criteria by reference to which coherence thus defined can be evaluated (the criteria are: (1) the number of supportive relations, (2) the length of the supportive chains, (3) the strength of the support, (4) the connections between supportive chains, (5) priority orders between reasons, (6) reciprocal justification, (7) generality, (8) conceptual cross-connections, (9) number of cases a theory covers, and (10) diversity of fields of life to which the theory is applicable). Such an approach raises many questions, such as how these various criteria of coherence are to be weighed and balanced against each other, and whether it is always the case that the weighing operation will result in a complete ranking of given sets of propositions as either more or less coherent than each other, so that when faced with competing such sets, it is always possible to find the most coherent set of propositions according to the ten criteria. Alexy and Peczenik recognise that weighing and balancing the criteria of coherence will be a complex matter, but appear to assume that it will always be possible to establish which is the most coherent of rival sets of propositions.