I'm aware of the phrase second nature:
a tendency or habit that has become characteristic or instinctive.
"deceit was becoming second nature to her"
Is there something that means "first nature"? Example sentence:
His light, like air, had become __ to her.
(Maybe "first nature" is common enough? And applicable in this situation?)
Note: By "first nature" I mean, things that are fundamental to us like food, shelter, air, etc.