Fantastic literature has been especially prominent in times of unrest, when the older values have been overthrown to make way for the new; it has often accompanied or predicted change, and served to shake up rational Complacency, challenging reason and reminding man of his darker nature. Its popularity has had its ups and downs, and it has always been the preserve of a small literary minority. As a natural challenger of classical values, it is rarely part of a culture's literary mainstream, expressing the spirit of the age; but it is an important dissenting voice, a reminder of the vast mysteries of existence, sometimes truly metaphysical in scope, but more often merely riddling.