There is no model of the world that is complete, consistent, and computable. Any intelligent system must encounter surprises.
Solutions to problems cannot be precomputed; many problems must be solved dynamically, starting from observed data.
Flexibility to deal with a variable environment requires search.
Ambiguity in interpretation of perceptual data requires search. Interpretation may be locally ambiguous, but global constraints may permit an unambiguous total interpretation.
Creativity can result from searching through many possible designs.