As a matter of principle

I’m doing a translation into Latin. I’m having trouble translating the phrase

(something is done) as a matter of principle.

The best I’ve come up with is

praecepti elementarii adseverandi causa.

(Which is a translation of my English re-phrasing of it: “for the sake of asserting a fundamental precept”)

Can anyone suggest something better. I guess there must be an actual equivalent already in Latin lterature. Anyone know what it is?