‘Impossible’ not limited to believe
Like most proverbs and aphorisms, the phrase has a grain of truth. But taken literally and applied universally, it’s completely misleading. For example, there’s just as much in its exact opposite: “Believing is seeing.” We “see” what we ourselves expect or project.
Philosophers and psychologists have known this for ages. We are not neutral observers of a totally objective, “out there” reality. We shape and influence the kind of life we have and our experience of the world by what we bring to it.
Thus, if you believe people are fundamentally hostile and the universe is a very dangerous place, that’s what you’ll perceive. The “evidence” will confront you everywhere. It reminds one of the paranoid agnostic who said, “My problem is I don’t know whether or not there’s a God. But there’s something out there and it’s out to get me!