Research on practitioner theories of advertising uncovered that agency practitioners not only have definite theoretical beliefs about how advertising works, they also have meta-theoretical beliefs, fundamental presuppositions about the nature and possibility of knowledge in advertising. The meta-theoretical belief in creativity and its dictum of 'no rules' was found to be more important than any other guiding principle in advertising work. The primacy of creativity denies the possibility of any other moderator-focused theories that would prescribe 'rules' for creative content. Practitioners believe that the ontological status of advertising (as a territory defined by creativity, art and tacit skill) places it mostly outside the reach of scientific modelling. While practitioners acknowledge that knowledge about advertising is 'layered' (i.e. certain aspects of it are more explainable by the legitimation system of science), they also insist that the creative 'layer' is much thicker than other layers. A further qualifying factor is practitioners' epistemological scepticism, which questions the validity of both academic and commercial social research as applied to advertising, and suggests instead that knowledge about advertising is better understood as 'common sense'. The study's findings have fundamental consequences for the professional aspirations of the advertising industry as well as the academician -practitioner gap in advertising.