When we write part of our life is exposed, by our experiences expressed, by the way we write what happens to us, and how it happens and also by so many other things that we capture in lines.

I can tell you that I have said things that “a posteriori” I don't remember that I said them, I have also written things that I don't really remember when I imagined them to be written. I don't remember the moment, I don't remember the handwriting I used and I don't remember why I wrote it.
I can tell you that it happens to me often. What I can faithfully tell you is that I don't plagiarize writings, I just forget that they came from my mind.