Damon Knight, author, editor, critic, and founder of the Science Fiction Writers of America, once attempted to quantify the elements of science fiction.

He came up with the following list derived from a number of previously published formal definitions of science fiction:

• science
• technology and invention
• the future and the remote past, including all time travel stories
• extrapolation
• scientific method
• other places--planets, dimensions, etc., including visitors from the above
• catastrophes, natural or manmade

Knight concluded that a story with at least three of the above elements is generally perceived to be science fiction; stories with two elements were borderline; stories with one or no elements were not science fiction ("What is Science Fiction?" in Bishop 4-5).