Precisely because the Constitution allows presidents to make non-corrupt mistakes in judgment, the true facts about Hunter Biden’s well-paid service on the board of a Ukrainian energy company or the vice president’s role in ousting a Ukrainian prosecutor don’t strictly matter. What does matter is Trump’s state of mind: What he actually believed, and what basis he personally had for that belief, should determine whether he committed an abuse of power.

All of which means that it’s Trump, and not the Bidens, who should take the stand here. Trump needn’t come to the well of the Senate for that; in deference to the presidency, he could be allowed to testify from the White House, the venue from which President Bill Clinton testified before independent counsel Kenneth W. Starr’s grand jury two decades ago.

But if Trump were to testify, it wouldn’t be enough for him to baldly assert that his motives were pure, or “perfect,” as he is wont to say. And even if he truly believes he acted in good faith, that wouldn’t be enough to acquit him, either. The president’s duty to faithfully execute his office includes not only a duty of loyalty to the nation but also a duty of care — a duty to act with reasonable diligence and upon a reasonable basis. President George W. Bush, for example, couldn’t have been impeached merely because he blundered into a war in Iraq. But Bush could be impeached if he decided to launch the invasion based on the advice of a Ouija board or a Magic 8-Ball.