Both Uses of

principle
in
The Princess Bride

Naturally, he countered with Capo Ferro which surprised the man in black, but he defended well, quickly shifting out of Agrippa and taking the attack himself, using the principles of Thibault.†
p. 147.1 *

principles = basic rules or beliefs

"There's such a thing as principle" was all Fezzik would answer, and he opened the door that led to the fourth level.†
p. 299.1

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)
meaning too rare to warrant focus:

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