The question they asked about Trump’s motive is not a question that is typically asked in a criminal trial. White collar criminal cases typically revolve around intent or knowledge, not motive. Juries are often asked to determine whether the defendant had the “intent to defraud” or acted “knowingly.”

At trial, the motive behind a defendant’s commission of a crime usually doesn’t matter. Even if a public official took bribes in order to pay his medical bills, he is still guilty of bribery. If a fraudster ripped off billionaires and gave the money to charity, she is nonetheless guilty of fraud.

But motive becomes an extremely relevant question at sentencing. Federal judges are required to consider all of the circumstances surrounding the offense, which includes the defendant’s motivations.

Pundits and politicians alike have analogized Trump’s impeachment trial to an actual criminal trial, and senators to actual jurors. By now, it should be clear how different the two proceedings really are. While this is a “trial,” the senators are not mere “jurors,” as Chief Justice William Rehnquist noted during the impeachment trial of President Bill Clinton. The senators set the rules, determine what evidence they want to hear, and ultimately determine whether the offense requires Trump’s removal from office and disqualification from serving in federal office going forward.

In other words, the senators are simultaneously acting as jurors and sentencing judges, voting on guilt at the same time that they administer the punishment. This is why Trump’s motives are relevant to senators as they consider whether his removal is warranted.

But discerning what happens in the mind of any human being can be difficult because there is no magic machine that allows us to peer inside their head. Typically, juries are asked to discern a defendant’s intent by looking at the defendant’s words and actions and by examining all of the evidence in the case. A judge looking to determine a defendant’s motive would do the same.

The debate thus far in Trump’s impeachment trial has centered around whether to call firsthand witnesses like former national security adviser John Bolton, who allegedly was told directly by Trump that he wanted to freeze aid to Ukraine until its officials helped investigate the Democrats, including his rival Joe Biden. Certainly, hearing from additional witnesses who could testify about Trump’s own words would be highly relevant.

But if senators really want to discern Trump’s motives, there can be no serious question that the best way to do that would be to hear from Trump himself. Any trial lawyer or criminal investigator will tell you that the best evidence of a person’s state of mind is his own words, and if Trump testified, senators could judge his credibility for themselves.

Frequently during the impeachment trial, lawyers on both sides of the trial have pointed to factual issues that have not been fully resolved by the record thus far. Trump’s testimony would fill in those gaps. For example, Romney wanted to know when Trump first ordered the aid removed. Trump could answer that question. He could answer a question asked by Murkowski and Collins about whether he had ever mentioned the Bidens as targets of his anti-corruption efforts in Ukraine before Joe Biden entered the 2020 race. How Trump answered those questions and many others could be weighed by senators as they consider whether his removal from office is warranted.

Trump wouldn’t be the first president to testify before Congress. Presidents Washington, Lincoln and Ford did so, though none of them testified in the context of impeachment. President Clinton previously testified under oath in a deposition, and senators considered that testimony. Whether the Senate could subpoena Trump and compel his testimony is an open question—President Truman fought a subpoena for testimony about his appointment of an assistant secretary of the Treasury and Congress backed off, although Congress has compelled testimony from former presidents.

But the Senate doesn’t need to go that far. They could simply invite Trump to testify and indicate that they would like to hear what he has to say. Trump would have the right to refuse, but he would do so knowing that the senators could consider his refusal to testify when they determine whether to vote to remove him from office.

In an actual criminal trial, jurors can’t consider a defendant’s refusal to testify because the Fifth Amendment protects defendants from being compelled to testify. You have the right to remain silent when you are in criminal jeopardy. Here, Trump is not accused of a crime—a point that his lawyers have made repeatedly.

Polls show that a clear majority of Americans from both parties agree that the impeachment trial “should allow witnesses with firsthand knowledge of the impeachment charges to testify.” No one has more firsthand knowledge of Trump’s intent and motivations than Trump himself.

Trump has made his anger and disdain toward the impeachment process clear, and it’s likely that he would decline the Senate’s invitation to testify. After all, the White House rejected a request for Trump’s lawyers to appear before the House Judiciary Committee in December. But Trump’s refusal to testify would itself be an important data point for senators to consider, and would make the need to hear from witnesses like Bolton more acute. In the end, the Senate cannot seriously claim to want to understand Trump’s motivations if senators don’t at least try to hear from Trump himself.