The principle of Intelligent Design is that biological organs and functions are too complex to have naturally arisen, therefore some intelligent higher being must have designed them. The issue however, is that is effectively the same as man's oldest religion: Animism.
Animism in the simplest form explains why and how things that can't be explained now happen. For instance, early man didn't understand rain, lightning, the seasons, much of the natural world. It's no wonder why the early Gods are related to nature.
Intelligent Design is effectively nothing more then modern day animism, where lightning and rain have been replaced with organic structures. What we cannot explain now, ergo Goddidit.
Discuss.