Conventional definitions of genres tend to be based on the notion that they constitute particular conventions of content (such as themes or settings) and/or form (including structure and style) which are shared by the texts which are regarded as belonging to them. Alternative characterizations will be discussed in due course. The attempt to define particular genres in terms of necessary and sufficient textual properties is sometimes seen as theoretically attractive but it poses many difficulties. For instance, in the case of films, some seem to be aligned with one genre in content and another genre in form. The film theorist Robert Stam argues that 'subject matter is the weakest criterion for generic grouping because it fails to take into account how the subject is treated' (Stam 2000, 14). Outlining a fundamental problem of genre identification in relation to films, Andrew Tudor notes the 'empiricist dilemma':