Clemens is what's known as a public figure. That means he has a very heavy burden to prove a defamation case.
Us ordinary citizens have an easier job. We only have to show two things--one that a false statement was made, and two, that the false statement was "published" to a third partry. And in a case of slander (oral defamation) we have to additionally show damage to our reputation as a result. In a libel (written defamation) case, all we have to show is that the written statement was false and that it was published--damages are presumed (it's called defamation per se) The amount of damages is naturally up to the jury to decide.
The law is different with "public figures", ie, politicians and celebrities. Under the doctrine of NY Times v. Sullivan, they must go beyond showing that a statement was false. They must also prove that the person made it maliciously (knowingly false with the intent to harm the defamed party), or with what's called "reckless disregard for the truth". Part and parcel of this burden of proof is demonstrating that the statement was false to begin with.
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