 <unk> and the combinatorial explosion . In 1972 Richard <unk> ( building on Stephen Cook 's 1971 theorem ) showed there are many problems that can probably only be solved in exponential time ( in the size of the inputs ) . Finding optimal solutions to these problems requires <unk> amounts of computer time except when the problems are trivial . This almost certainly meant that many of the " toy " solutions used by AI would probably never scale up into useful systems . 
