Ah the "I" word. It is more than you think, as this edition of the right and Hon. Judge M. Hirsch's Constitutional Calendar teaches:
Thomas Jefferson swept into presidential office determined to purge the federal judiciary of the Federalist judges appointed by his two predecessors. His weapon would be impeachment. He would start with the low-hanging fruit and, he hoped, work his way up to the greatest prize of all: his bitter antagonist and distant kinsman, Chief Justice John Marshall.
The very lowest-hanging fruit was John Pickering of the U.S. District Court for the District of New Hampshire. Pickering, once a leading member of the bar and the author of his state's constitution, had degenerated into dementia and alcoholism. On February 4, 1803, the president transmitted certain "letters and affidavits" to the House of Representatives with his encouragement that impeachment proceedings be instituted. They were.
Trial before the Senate began on March 4, 1804. But the Senate was immediately presented with a problem of constitutional interpretation. Impeachment required that Pickering, like anyone to be impeached, be guilty of high crimes and misdemeanors. The evidence that at all times material Pickering was insane and incapable of forming the intent to commit any crime was irrefutable. In an ordinary criminal proceeding, this would render conviction impossible. But surely Pickering was not entitled to remain in office as a United States district judge because he was too demented to form criminal intent.
The senators reconciled themselves to the notion that the "misdemeanors" referred to in the demised constitutional provision did not, or did not necessarily, refer to criminal misdemeanors, but was simply a catch-all phrase for misbehavior inconsistent with the duties of office. On that basis, Pickering was convicted.