It's used this way: "At one time, people believed that [insert old theory here]. We now know that [insert new theory here]."
Makes you wonder, what are we going to "know" tomorrow that will change what we "now know" into what we "once believed"?
KAY: Any given time, around fifteen hundred landed aliens are on the planet, the majority right here in Manhattan. Most aliens are decent enough, just trying to make a living.EDWARDS: Cab drivers?
KAY: Not as many as you'd think. Humans, for the most part, don't have a clue. Don't want one, either. They're happy. They think they've got a pretty good bead on things.
EDWARDS: Why the big secret? People are smart, they can handle it.
KAY: A person is smart. People are dumb. Everything they've ever "known" has been proven to be wrong. A thousand years ago everybody knew as a fact, that the earth was the center of the universe. Five hundred years ago, they knew it was flat. Fifteen minutes ago, you knew we humans were alone on it. Imagine what you'll know tomorrow.
EDWARDS: So what's the catch?