John M. Barry

In fact, biology is chaos. Biological systems are the product not of logic but of evolution, an inelegant process. Life does not choose the logically best design to meet a new situation. It adapts what already exists...The result, unlike the clean straight lines of logic, is often irregular, messy.

Another explanation for the failure of logic and observation alone to advance medicine is that unlike, say, physics, which uses a form of logic - mathematics - as its natural language, biology does not lend itself to logic. Leo Szilard, a prominent physicist, made this point when he complained that after switching from physics to biology he never had a peaceful bath again. As a physicist he would soak in the warmth of a bathtub and contemplate a problem, turn it in his mind, reason his way through it. But once he became a biologist, he constantly had to climb out of the bathtub to look up a fact.