The text is from a book on how to hypnotize people. When hypnotizing, you'll often say things in a particular even tone without thinking too much about grammar or word choice. You want to keep the subject relaxed and focused on themselves, and not necessarily on what you are saying.
The line is from a sample script on how to hypnotize a subject by activating certain emotions:
"What would you say the opposite to 'nervous' is? Calm? Contented? Anything else?"
"Calm would be fine."
"Right. We will just keep your mind on the simple idea of being calm then." (Pause). "Calmer and calmer." (Pause.) "Calmer and calmer" (Pause.) "Don't hurry or worry. Just keep the idea of calmness pure and simple grow." (Pause.) "Calmer and calmer." (And continue on these lines for a few minutes or more.) "Now how do you feel?"
As a hypnosis subject, the weird phrasing would probably take me out of any calm state, as I pondered what the hypnotist was trying to say For me, it would have been better to say something like:
"Just let the idea of calmness, pure and simple, grow within you."
Then it's clear "pure and simple" are adjectives that modify "calmness", which the hypnotist wants me to "let grow".