Modernism is not a term to which a single meaning can be ascribed. It may be applied both to the content and to the form of a work, or to either in isolation. It reflects a sense of cultural crisis which was both exciting and disquieting, in that it opened up a whole new vista of human possibilities at the same time as putting into question any previously accepted means of grounding and evaluating new ideas. Modernism is marked by experimentation, particularly manipulation of form, and by the realization that knowledge is not absolute. Marx, Freud, and Darwin had unsettled the human subject from its previously secure place at the centre of at least the human universe, and had revealed its unwitting dependence on laws and structures outside its control and sometimes beyond its knowledge. Historical and material determinism, psychoanalytic theories which reveal the self as a pawn in a process dominated by an inaccessible unconscious play of forces, and a conception of evolution and heredity which situates humanity as no more than the latest product of natural selection—these theories conspired to threaten humanist self-confidence and to provoke a feeling of ideological uncertainty.