Both Uses of

principle
in
The Grapes of Wrath

The house was dead, and the fields were dead; but this truck was the active thing, the living principle.†
Chpt 10 *

principle = basic rule or belief

In their lapels the insignia of lodges and service clubs, places where they can go and, by a weight of numbers of little worried men, reassure themselves that business is noble and not the curious ritualized thievery they know it is; that business men are intelligent in spite of the records of their stupidity; that they are kind and charitable in spite of the principles of sound business; that their lives are rich instead of the thin tiresome routines they know; and that a time is coming when they will not be afraid any more.†
Chpt 15

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.