Passage Summary: The author uses an analogy: just as you need engineering (not just physics) to understand what a machine is for, you need physiology (not just chemistry) to understand how a body works.

Conclusion: Physiology is uniquely qualified to analyze organisms based on the functional roles of their organs.

Reasoning: Just as engineering is the only field that can analyze machines via purpose (while physics and chemistry cannot), physiology is the only field that can analyze biological operational principles.

Analysis: This argument relies on a 'Gap' between the two sides of the analogy. For the comparison to hold, the author must assume that the relationship between physics/chemistry and machines is the same as the relationship between physics/chemistry and organisms. Specifically, the argument requires the assumption that physics and chemistry are just as limited in the biological realm as they are in the mechanical realm. Look for an answer that confirms physics and chemistry cannot express the 'operational principles' or 'healthy functioning' of an organism.