Source of Piet Hein quote

Problems worthy
    of attack
prove their worth
    by hitting back.
        ---Piet Hein


"Problems," Grooks,
Borgens Forlag, Copenhagen, Denmark, The MIT Press, Cambridge, MA, 1966;
Grooks 1, Doubleday & Company Inc., Garden City, NY, General Publishing Company Limited, Toronto, 1969;
Collected Grooks I,
Borgens Forlag, Copenhagen, Denmark, 2002

to Quotes selected by Palle Jorgensen

Problems worthy of attack prove their worth by hitting back. ---Piet Hein

Problems worthy
of attack
prove their worth
by hitting back.
---Piet Hein