once the mind has been exposed to knowledge, it can't go back to what it was.

Indeed you can't unknow something, but you can ignore what you know and make use of it. Eventually knowledge can fade and have less influence, and you can even forget. It's happened to me and then I come across something and I'm like oh yeah I knew this too but from many years ago and I forgot, or it's influence in my recollection was diminished to the point I never recalled it when thinking about things.