Anyone, anywhere could modify x, which is what makes reasoning about this code difficult. When your entire program is a collection of mutable state that can be

Anyone, anywhere could modify x, which is what makes reasoning about this code difficult. When your entire program is a collection of mutable state that can be mutated by any piece of code, understanding what's going on becomes really difficult if not impossible. One thing that people are adopting from FP is to limit what is mutable, and when possible make state explicit.