If you are a private figure and The New York Times or the Southern Poverty Law Center publishes a lie about you, you simply have to prove that the statement was false and harmed your reputation. The fact that the publisher didn’t know or care that the statement was false is irrelevant.
But if you are a “public figure,” you not only have to prove that the statement was false and harmed your reputation, but that the statement was made “with knowledge that it was false or with reckless disregard of whether it was false or not.”
And the definition of who is a public figure constantly has expanded since 1964.