I found this cool video from TedEd about why we are so attached to our possessions. It seems that we literally take the things that we possess to a part of ourselves on a neural level. So it appears that the Buddha’s teaching on not taking anything external to be our selves is based on measurable science.
“So you should truly see any kind of form at all—past, future, or present; internal or external; coarse or fine; inferior or superior; far or near: all form—with right understanding: ‘This is not mine, I am not this, this is not my self.’ SN 22.59
So taking any external object to be a part of our self, or identifying an essence with an external object as a part of our (or someone else’s) self is a very real thing that we do. Even if it’s not something that is superstitious sounding like a horcrux or voodoo doll. Your favorite coffee mug might be something that you consider a part of you! It’s really neat, and slightly alarming, to consider.
If you would like to watch the video the link is below. Video length is around 4.34 minutes.
After witnessing the “violent rage” shown by babies whenever deprived of an item they considered their own, Jean Piaget – a founding father of child psychology – observed something profound about human nature: Our sense of ownership emerges incredibly early. But why do we become so attached to things? Christian Jarrett details the psychology of ownership.
There is more information and papers cited on how we identify objects as our self here from TedEd.