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.