The word principle is a common noun, a general word for any kind of principle.
A proper noun is the name of a person, place, thing, or a title; for example:
- The Peter Principle, formulated by Dr. Laurence J. Peter in his book called 'The Peter Principle'
- Principle Road, Camarillo, CA
- Principle Graphics Ltd., Northhampton, UK
- 'The Domino Principle' (1977 movie) with Gene Hackman