Apparently simple, but really rather deep

Computing pioneer Maurice Wilkes, in a letter to Tony Hoare from August 1981:

. . . loop invariants . . . The idea is apparently simple, but really rather deep.  Once you have grasped it fully – and I must confess that, although Floyd was very patient with me, it was some time before I did this – you will never look at the subject in the same way again.

Quoted in C. B. Jones, The Early Search for Tractable Ways of Reasoning about Programs, IEEE Annals of the History of Computing, Vol 25, No 2, pp26-49, 2003