Your definition falls down when faced with real world problems. I can write a 'perfect' chess program in less than a day that could easily run on a PC, however it's run time would make it useless in the real world. Crossing the Atlantic is an easy problem now, but that does not mean it was easy 500 years ago even though mathematically nothing has changed. And in 500 years computer vision is probably going to be considered a vary easy problem often done as a high school level project, but that does not mean it's easy now.