I used to have a t-shirt that I got from Primavera. It had an illustration of a artist, a scientist and a large bull. The caption was "Project Management - Art, Science or ...? If nothing else it was an acknowledgement that Project Management is at best an amalgam of the three. It is left as an exercise for the reader to determine the relative proportions in any particular "theory" or approach. Coming from a design background I can't help but recommend this article in Design Observer on the same topic.