Both Uses

principle
in
To Kill a Mockingbird
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He was a year older than I, and I avoided him on principle: he enjoyed everything I disapproved of, and disliked my ingenuous diversions.†
p. 88.8 *

on principle = because of a basic rule or belief
Not that we'd ever done anything, it was the principle of the thing.†
p. 289.6

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.