Both Uses

principle
in
The One and Only Ivan
(Edited)

For example, it's against my principles to do my daughter's homework for her.
p. 195.3 *

principles = basic rules or beliefs of what is right
It's a matter of principle, Dad.
p. 219.8

principle = basic rule or belief

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