>what we call personality is exactly that which is unchanging in the face of a dynamic environment

I am of the exact opposite opinion, given how much peoples personalities can change given input (think of someone spending all their time alone, and being depressed, changing their lifestyle towards going out every few days to hang out with new friends, and reportedly now having a "cheery personality", or simply someone with no passions discovering theirs, and then feeling like there is meaning to life).