Each success only buys an admission ticket to a more difficult problem. Henry Kissinger
If you procrastinate when faced with a big difficult problem... break the problem into parts, and handle one part at a time. Robert Collier
To select an appropriate fuzzy implication for approximate reasoning under each particular situation is a difficult problem. Although some theoretically supported guidelines are now available for some situations, we are still far from a general solution to this problem. George Klir
It is many years since I have wielded a weapon larger than a pen, borne anything weightier than a difficult problem in philosophy. Michael Moorcock
Every victory is only the price of admission to a more difficult problem. Henry Kissinger
Power is a very difficult problem with which to deal in the theory of organization. Michel Crozier