An impeachment trial is overseen by Congress and Senate, who both make the law and (in this case) sit in judgment on it, ignoring the modern legal principle of the separation of powers. Athenians would have been shocked, not because they believed in the separation of powers (their citizens too made the law in assembly and sat in judgment in the law courts) but because in democratic Athens it was citizens that decided verdicts, and in randomly selected and therefore unpredictable juries.