Which belief or idea is the most yours? The one that has the least external influence and is the purest thought of your being.

This question has been in my head the last few days, it's pretty personal and I have tried to answer it but I find it hard to approach, I don't have idea about how to get started. The premise for this question would be that there is a range of influence in your ideas and beliefs hence there should exist a minimum. The main reason of this post is to ask about similar questions that different authors have made and may be helpful to find an answer to it. Anything would be appreciated.