On sculpture – typically for him, colourless form – generates in the mind the idea of shape which we must conceptualise, as with 'proportion'. The literary arts, like sculpture, deal with thoughts and emotions, though in a more complex form. Knight's account of these arts therefore falls under the heading of 'association of ideas'. Here Knight shows the influence of the contemporary cult of sensibility, arguing that these arts engage our sympathies, and in so doing demonstrate the inadequacy of 'rules and systems' in both morality and aesthetics. These teach 'men to work by rule, instead of by feeling and observation.' Rule-based knowledge of wrong cannot prevent wrongdoing, because it is thought not felt. Therefore, 'it is impossible that tragedy should exhibit examples of pure and strict morality, without becoming dull and uninteresting.’