The Encyclopedia of Science Fiction, edited by John Clute and Peter Nicholls, contains an extensive discussion of the problem of definition, under the heading "Definitions of SF". The authors regard Yugoslav born academic, writer, and critic Darko Suvin's 1972 definition as having been most useful in catalysing academic debate. Suvin's definition is: "a literary genre whose necessary and sufficient conditions are the presence and interaction of estrangement and cognition, and whose main formal device is an imaginative framework alternative to the author's empirical environment".[7]