"Difficult things take a long time;
 the impossible takes a little longer."
 - Chaim Weizmann

"Nothing is more impossible than to write
 a book that wins every reader's approval."
 - Miguel de Cervantes

"When a distinguished but elderly scientist states that something is possible,
 he is almost certainly right.  When he states that something is impossible,
 he is very probably wrong." - Clarke's First Law