Only engineering is capable of analyzing the nature of a machine in terms of the successful working of the whole; physics and chemistry determine the material conditions necessary for this success, but cannot express the notion of purpose  Similarly, only physiology can analyze the nature of an organism in terms of organs' roles in the body' s healthy functioning
Physics and chemistry cannot ascertain by themselves any of these operational principles
The notion of purpose used by engineers to judge the success of machinery has an analog in organisms.