The Senate must answer just two questions at President Trump’s impeachment trial. First, did he do either of the things the articles of impeachment allege he did: abuse his presidential power or obstruct a congressional investigation? If the answer to that question is yes, the Senate must then ask whether that conduct warrants impeachment’s main penalty: removal from office.

But the confusion and arguments over how the trial will proceed are unending. Last week, some Republican senators proposed a solution to the impasse over whether the Democratic House managers arguing the case against Trump can offer witnesses at the trial. They suggested calling witnesses in pairs — a Republican witness for every one the Democrats are permitted. “We’re not going to have a one-sided kangaroo court,” Sen. Ted Cruz (R-Tex.) said in explaining his proposal. “Instead, we’re going to respect reciprocity. What does that mean? That means, if the prosecution gets a witness, the defense gets a witness. If the prosecution gets two, the defense gets two. That means, if the prosecution gets to call John Bolton, then the president gets to call Hunter Biden.”

The problem is, no such concept of “witness reciprocity” exists in the U.S. justice system.

Reciprocity “would terrify Democrats,” Cruz said, because, according to him, they fear there would be evidence of “actual corruption” if Biden were called to testify. Leave aside for the moment that no evidence of illegality by Biden has turned up, and Trump’s Justice Department has not charged anyone, which it could presumably do if such evidence existed. The implication was that reciprocity might discourage Democrats from pursuing witness testimony. Some commentators took up Cruz’s challenge, however, saying Democrats should be willing to offer up Hunter Biden, or even his father, Joe Biden, Trump’s possible rival in the 2020 presidential race, to get the testimony of firsthand witnesses such as Bolton, Trump’s former national security adviser; acting White House chief of staff Mick Mulvaney; and others during the Senate proceedings.

The temptation to secure the testimony of key witnesses is understandable, but “witness reciprocity” is not how our system of justice works, and for sound reasons. It might seem like a good idea at first: Treat each side equally, and give them the same number of witnesses. But in our system, evidence must be relevant to the charges or issues at hand before it can be introduced in a trial. To be relevant, evidence must tend to make the existence of any fact that is necessary to deciding the case more or less likely than it would be if the evidence was unavailable to the decision-maker. The rule of relevance ensures that cases are decided based on evidence, not distraction or tricks. It compels focus on the issues in the case being decided, not any others.

For example: Some people might want to hear testimony from adult-film star Stormy Daniels during the impeachment trial. Some might want to see Trump’s tax returns. Neither will happen, because the testimony is not relevant to the charges on which the House impeached the president. Similarly, Trump should have the right, even if he chooses not to exercise it, to call witnesses whose testimony helps establish that he didn’t commit the offenses alleged in the articles of impeachment or that they’re not impeachable acts. But he shouldn’t be able to go far beyond that. He couldn’t, for instance, argue that the Ukrainians weren’t entitled to military aid from the United States. That’s irrelevant, because Congress had already voted to send it to them.

The same goes if Republicans subpoena Hunter Biden. His story is unrelated to whether Trump committed the acts for which he was impeached. Even if Biden behaved corruptly in Ukraine, proof of that would not absolve Trump of charges that he abused his presidential power. And Biden’s conduct has no bearing on whether the president withheld witnesses and evidence from Congress, obstructing its investigation. Biden’s testimony wouldn’t make it more or less likely that Trump abused his power or obstructed Congress. It is simply not germane.

Calling Biden as a witness could suggest only that his role on the board of the Ukrainian gas company Burisma while his father was U.S. vice president somehow justified Trump’s alleged plan to coerce an announcement out of Ukrainian President Volodymyr Zelensky to investigate. Yet there isn’t a scintilla of evidence to support Trump’s suggestion that Biden violated the law, and again, Trump’s Justice Department has not pursued charges against him.

Moreover, Trump’s sole demonstration of a personal interest in fighting foreign corruption is tied to the Bidens — and by extension his political future. The president has shown no interest in fighting corruption in other regimes and has embraced leaders in countries where corruption is rife, such as North Korea, Russia and the Philippines. On the contrary, rather than demonstrating a desire to fight corruption abroad, Trump has shown a desire to fight for it. In their new book, “A Very Stable Genius: Donald J. Trump’s Testing of America,” Washington Post reporters Philip Rucker and Carol D. Leonnig write that Trump clashed with then-secretary of state Rex Tillerson over the need for the Foreign Corrupt Practices Act, a key law that prevents U.S. companies from participating in corrupt business practices abroad. “It’s just so unfair that American companies aren’t allowed to pay bribes to get business overseas,” Trump said, according to the book, adding, “We’re going to change that.”

In Ukraine, Trump’s only concern involved the Bidens and Burisma, despite his stated view that the country was “totally corrupt.” Nor did the president seem to be interested in an investigation that could aid U.S. law enforcement in bringing wrongdoers to justice. His demand was only for the announcement of an investigation. He has apparently not forgotten how devastating an announcement alone can be to a campaign.

There are legitimate and well-established ways for the Justice Department to engage with law enforcement in other countries in appropriate cases. They do not include the president’s conditioning the release of congressionally approved security aid to a strategic partner on “a favor.” Last week, the Government Accountability Office, a nonpartisan government agency, concluded that Trump had violated the Impoundment Control Act when he held up aid to Ukraine.

Hunter Biden’s behavior in connection with Ukraine has no bearing on the issues raised by the articles of impeachment. He is simply, as lawyers say, a red herring. He cannot testify to anything that sheds light on either of the questions the Senate must answer. Nothing he says would make it more or less likely that Trump committed impeachable acts. Trials are about providing decision-makers with evidence that is relevant to the matter at hand so they can arrive at the truth. Alternative facts should play no role in an impeachment trial, only real ones.

Democrats should not take up a Republican offer, if it is made, to play a game of witness reciprocity. Trial evidence is not about tit for tat. While the John Boltons of this administration can offer relevant, firsthand information regarding Ukraine and the president’s conduct and must testify if the proceedings are to have any integrity, the Hunter Bidens of the world cannot.