As most would agree, Modernism is very difficult to define. All movements have fuzzy borders. Moreover, some who would be defined as modernists by others disavow that title for themselves.
Though this approach is somewhat reductionistic, a few principles apply. First, Modernists rejected the past, especially a totalizing view of the world or a grand narrative. Specifically, they rejected the 19th century – the Victorian era.
Second, Modernists rejected an epistemology of any absolutes. Therefore, post-Enlightenment thoughts of absolutes were long gone. World War I certainly helped shatter this thought.
Third, Modernists were influenced by Freud’s psychoanalytic theories. For example, Modernists sought to experiment and create new ways of seeing and writing. For example, some poets used things like the Stream of Consciousness technique to portray the way the human works. The quote that sums up the basic spirit of Modernism is Ezra Pound's famous statement: "Make it new."