What do you think the secret to Think and Grow is?

In Think and Grow Rich, Napoleon Hill eludes to "a secret which is mentioned in every chapter" but never specifically mentioned..

He says the point is to discover it for yourself and when you do, it will click within your brain and life will never be the same..

.. so, do YOU know what it is? What's YOUR theory?

Do you think it is one of the chapters in particular, or that it is a core principle outside of this?

I have my own theory but don't really want to say just now, just thought it would be a fun exercise to find out what your theories are.

Please share your theory in the comments below.. I think I'm onto something, but I really want to hear what other people think... and maybe we can figure it out collectively and all benefit :)

PS. Here's my take on the secret of Napoleon Hill's book.