Answer: because even a layman understand how energy works
Explanation:
The first law of thermodynamics states that energy can not be created or destroyed but can be transformed from one form into another. This is called a natural principle because a layman knows that energy is transformed from one form to another but do not understand why and how which needs scientific explanation.
Example
A layman knows that plant need sun (solar energy) to produce their food (chemical energy). A non scientists know that electrical energy can do work (mechanical energy), electrical energy can produce heat but they do not have scientific explanation for it.
