Learn English – What exactly is a “principle of action” or a “principle of conduct”

historymeaningobscure-termssemantics

Initial Context

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)

Question

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."
Occurrence of "principal of action" and "principal of conduct" in the Google N-grams Corpus, 1500-2008

Sources

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.

Best Answer

The phrases "principle of action" and "principle of conduct" don't seem like idioms to me, nor divorced from their constituent words.

Rather your examples seem to me to be straightforward applications of the word principle. From the OED's entry for principle, for example, we read:

I.b: A fundamental source from which something proceeds.

II.: Fundamental truth or law; motive force.

II.3.a: A fundamental truth or proposition on which others depend ... forming the basis of a system of belief ; a primary assumption forming the basis of a chain of reasoning.

II.3.b: A primary proposition, considered self-evident, upon which further reasoning or belief is based.

II.4.a: A general law or rule adopted or professed as a guide to action; a settled ground or basis of conduct or practice; a fundamental motive or reason for action, esp. one consciously recognized and followed.

II.4.b: A personal code of right action; rectitude, honourable character.

II.7.a: A general fact or law of nature by virtue of which a machine or instrument operates; a natural law forming the basis of the construction or operation of a machine, appliance, etc.

The single most applicable sense here is 4a, but the general gloss of a "fundamental motivating force" is the sense in play in both "principle of action" and "principle of conduct" (though the latter has a stronger sense of ought as in the prescriptive 4b, whereas the former has a stronger sense is, as in the descriptive 7a).

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