Solaris editors, I'm looking at you here (if you exist, which is a separate question):
"Is it principal or principle ? Principal means 'most important' or 'person in charge': : my principal reason for coming tonight;: the high school principal. It also means 'a capital sum': : the principal would be repaid in five years. Principle means 'rule, basis for conduct': : her principles kept her from stealing despite her poverty."
Principle is always a noun, never an adjective. Principal could be either. But you CANNOT have a "principle affinity."
(Usage text in quotation marks above courtesy of the dictionary app on my MacBook Pro.)