Fantasy is an exercise bicycle for the mind. It might not take you anywhere, but it tones up the muscles that can.

Terry Pratchett (1948-2015) English author

Interview in Leonard Marcus, The Wand in the World: Conversations with Writers of Fantasy (2006)
    (Source)

This quotation is sometimes given with "But I may be wrong" as a following sentence, but that does not appear in the original.