Rationale
Part of speech: noun
Definition: an explanation of the basis or fundamental reasons for something a justification or rationalization for something a liturgical vestment worn by Christian bishops of various denominations
Example sentence: Surely, the best and most effective measure is to get the economy moving and shorten the period of recession or slowdown. That's the rationale for Gordon Brown's 'fiscal stimulus' and it sounds like a good one to me.
Principle
Part of speech: noun
Definition: A fundamental assumption.A rule used to choose among solutions to a problem.Moral rule or aspect.A rule or law of nature, or the basic idea on how the laws of nature are applied.A fundamental essence, particularly one producing a given quality.A beginning.
Example sentence: Trust is the glue of life. It's the most essential ingredient in effective communication. It's the foundational principle that holds all relationships.