George Washington University Law School professor Jonathan Turley testifies during a hearing before the House Judiciary Committee on the constitutional grounds for the impeachment of President Donald Trump, on Capitol Hill in Washington, Wednesday, Dec. 4, 2019. From left, Harvard Law School professor Noah Feldman, Stanford Law School professor Pamela Karlan and University of North Carolina Law School professor Michael Gerhardt. Credit: Alex Brandon | AP

Maybe House Judiciary Committee Chair Jerrold Nadler didn’t choose the best word when, in his opening statement before the committee’s first public hearing last week, he called the facts connected with the Trump impeachment inquiry “undisputed.”

Committee Republicans took immediate and indignant exception to Nadler’s characterization, and, indeed, they publicly asserted that they were herewith disputing the facts.

In one sense the Republicans are correct: All facts are theoretically disputable. We concluded long ago that, despite appearances, our earth is round. Yet the Flat Earth Society maintains a well-appointed website that argues, apparently seriously, that the earth is a flat plane rather than a globe.

But at least Nadler’s characterization of the facts in the impeachment inquiry provides the occasion to consider the nature of a fact, as well as the difference between evidence and proof.

Evidence and proof do not reside on opposite sides of a clear bright line. They generally are divided by a broad no-man’s land filled with bias and emotion, and fair-minded people can honestly disagree on where a “fact” — disputed or not — resides on the spectrum between evidence and proof.

This point is exemplified by the disagreement among the four legal scholars who testified before the Judiciary Committee last week. All four looked at the same evidence. Three of them (Noah Feldman, Pamela Karlan and Michael Gerhardt) were convinced that the evidence had migrated across the line into proof. Feldman said, “Based on that evidence and those findings, the president did commit an impeachable abuse of office.”

Jonathan Turley, however, did not agree. He called the evidence gathered by the House Intelligence Committee “wafer-thin” and suggested that more evidence is needed before a credible case for impeachment can be pursued.

The contrast between these two views of the same evidence implies the extent that bias, preconceived ideas and emotions, from both liberals and conservatives, can affect the rational evaluation of facts.

Nevertheless, despite Republican assertions to the contrary, some facts are indeed indisputable.

For example, during the July 25 phone call President Donald Trump asked Ukrainian President Volodymyr Zelenskiy for a favor. This is impossible to dispute: The transcript of the call released by Trump says, “I would like you to do us a favor though.”

What is the favor? Trump doesn’t actually say, “The favor is as follows…” But Trump clearly appears to ask Zelenskiy to start two investigations that would benefit Trump’s electoral prospects in 2020. This proposition may not depend on a quid pro quo, but it does at least sound like a violation of federal election law, which prohibits even the solicitation of foreign interference. Feldman, Karlan and Gerhardt found this evidence credible; Turley found it insufficient.

Here’s another undisputed fact: Ambassador Gordon Sondland testified under oath that the promised White House meeting and the $391 million in security aid were dependent on a quid pro quo and that “everyone was in the loop.” The fact of his testimony is indisputable. But, again, Feldman, Karlan and Gerhardt found Sondland credible; Turley did not.

Accounting for the disagreement among qualified experts about evidence deriving from undisputed facts implies the power of bias. But in some respects, the American people’s assessment of the evidence is more important than the judgments of four law professors. The viability of the effort to impeach Trump will depend greatly on public opinion and, thus, every American has a responsibility to make a careful assessment of the evidence.

The fact that I find Turley less credible than Feldman, Karlan and Gerhardt may reflect my own prejudices. But Turley was right about one thing: his argument that the evidence is insufficient to constitute proof implies that more evidence is still available. Indeed, Turley counseled taking more time and calling more witnesses.

Here’s one more undisputed fact: People who know a lot about what happened between Trump and Zelenskiy — Bolton, Pompeo, Perry, Mulvaney, Giuliani — have been forbidden by Trump from testifying. Does this prove anything about Trump’s guilt? No. But it’s more evidence to which Americans are entitled. The withholding of the evidence is yet another deeply suspicious undisputed fact.

John M. Crisp, an OpEd columnist for Tribune News Service, lives in Georgetown, Texas.