"Eternal"?

childhood, existence.

Nope.

poison

pain
body
soul
spirit

life
love
security
inconceivability

capital
revenge
payment
revenue

Any of these because none of the hint added up...
And what does it mean in the 1st hint when it says "blank [adjective]" < That showed no relevance to the question

Nope.

You have to find the adjective in john locke's literature.

commonsense?

forgetfulness?

Nope.

reality?