What's wrong with the rational model We Don’t Really Know the Goal When We Start We Usually Don’t Know the Decision Tree – We Discover It as We Go The Nodes Are Really Not Design Decisions, but Tentative Complete Designs The Goodness Function Cannot be Evaluated Incrementally The Desiderata and Their Weightings Keep Changing The Constraints Keep Changing Frederick P. Brooks, Jr., What's Wrong With This Model? Changing constraintsDeciding what to designThe situation talks back