I was reading one of John Henry Newman's (Cardinal Newman for the non-Anglicans) sermons, specifically "Religious Faith Rational" from Parochial and Plain Sermons...
Near the beginning of the text he writes
But it is not at all true that Faith itself, i.e. Trust, is a strange principle of action; and to say that it is irrational is even an absurdity.
Later in the text he echoes this, writing
it is no irrational or strange principle of conduct in the concerns of this life.
I was intrigued by and confused as to the dictionary (or encyclopedia, handlist, glossary, etc...) definition of this phrase (term of art?). When I did a Google search I found the phrase in a not-insignificant amount of philosophy related texts (viz. those on Bentham and Kant)
What exactly is a “principle of action” or a “principle of conduct”? Are these terms interchangeable? In what situations are the terms uses, historically or contemporaneously?
Visualisation of Frequency
I did an n-grams search and it looks like the term was in current use in 1829, when Newman wrote "Religious Faith Rational."
Michel, Jean-Baptiste et al. “Quantitative Analysis of Culture Using Millions of Digitized Books.” Science 331.6014 (2011): 176–182. www.sciencemag.org. Web. 15 Nov. 2014.
Newman, John Henry, and William John Copeland. Parochial and Plain Sermons. New ed. Vol. 1. London, New York: Longmans, Green, 1891. The Open Library. Web. 15 Nov. 2014. 8 vols.