All 5 Uses of

principle
in
Light in August

It was as if he struggled physically with another man for an object of no actual value to either, and for which they struggled on principle alone.†
Chpt 11

on principle = because of a basic rule or belief

Decently hung by a Force, a principle; not burned or hacked or dragged dead by a Thing.†
Chpt 19 *

principle = basic rule or belief

Though when he learned that the Republicans did have a name for it, he completely changed the name of his conviction without abating his principles or behavior one jot.†
Chpt 20

principles = basic rules or beliefs

The very fact that he could and did see no paradox in the fact that he took an active part in a partisan war and on the very side whose principles opposed his own, was proof enough that he was two separate and complete people, one of whom dwelled by serene rules in a world where reality did not exist.†
Chpt 20

He lived by his principles in peace, and when war came he carried them into war and lived by them there; when there was preaching on peaceful Sundays in quiet groves to be done, he had done it, without any particular equipment for it other than his will and his convictions and what he could pick up as he went along; when there was the saving of wounded men under fire and the curing of them without proper tools, he did that too, again without other equipment save his strength and courage and what he could pick up as he went along.†
Chpt 20

Definitions:

(1) (principle)
a basic rule or belief
The exact meaning of principle can depend upon its context. For example:
- "our guiding principles" -- basic moral beliefs that guide decisions and behavior
- "electromagnetic principles" -- rules describing how the world works
- "She lacks principles." -- lacks moral guidelines
- "We agree in principle." -- about important basic beliefs
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

See a comprehensive dictionary for more specialized senses including one in chemistry.