in the author's preface what is the basis for shelley's story? in other words why did she write this novel

Shelley and her romantic poet friends were vacationing at a home on Lake Geneva. , Lord Byron, yes Byron the poet, suggested they pass the time by writing a ghost story. Shelley notes,

"I busied myself to think of a story... a story to rival those which had excited us to this task. One which would speak to the mysterious fears of our nature, and awaken thrilling horror -- one to make the reader dread to look round, to curdle the blood, and quicken the beatings of the heart,"

Shelley claims she developed the story from a dream she had.