An ontology allows elements to be assigned to more than one class. This is useful, for instance, for areas (of which there are many in public health) on which there is little agreement and competing views. In a classification system, with its emphasis on mutually discrete classes, it is not so easy to do this. 84 Noy & McGuinness 2001: 4. 85 The wicked problem concept in design was described by Rittel & Webber (1984) in the context of social planning. They pointed out that in solving a wicked problem, the solution of one aspect may reveal another, more complex problem. Ten rules define the form, including that there is no definitive formulation of a wicked problem (no stopping). Solutions to wicked problems are not therefore true-or-false, but good-or-bad.