Prediction is very difficult, especially of the future.

Related:

• It is very difficult to prophesy,
  especially when it pertains to the future...
• It is very difficult to prophesy,
  especially when it pertains to the future...
• The art of prophecy is very difficult, especially with respect to the future.
  Mark...
• An expert is a man who has made all the mistakes which can be made,
  in a very narrow field. -- Niels...
• Making predictions is always difficult,
  especially about the future...
• Accuracy and clarity of statement are mutually exclusive.
  Niels Bohr (1885...
• Albert, stop telling God what to do.' --
  Niels...
• A physicist is just an atom's way of looking at itself.
  Niels...
• There are some things that are so serious that you have to laugh at them.
  Niels...