Both Uses of

principle
in
The Screwtape Letters

The congregational principle, on the other hand, makes each church into a kind of club, and finally, if all goes well, into a coterie or faction.†
Chpt 16 *

principle = basic rule or belief

We are certain (it is a matter of first principles) that each member of the family must in some way be making capital out of the others—but we can't find out how.†
Chpt 22

principles = basic rules or beliefs

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.