Of course, the main reason witnesses (and everyone else) should tell the truth is because it's the right thing to do. But the advice from prosecutors gets at a different point. Lies beget more lies, and more lies generate more confusion, and soon the lies cause more trouble than whatever caused the controversy in the first place. This phenomenon is at the heart of President Donald Trump's current legal problems.
Consider the matter of Stephanie Clifford, also known as Stormy Daniels, who is now America's most famous adult film performer. She asserts that she and the President had sex once in 2006. Five years later, a supermarket tabloid recounted their alleged sexual encounter.
And five years after that, at the climax of the 2016 presidential campaign, Michael Cohen, Trump's lawyer and fixer, arranged for a shell corporation to pay Daniels $130,000 for her agreement to remain silent about her relationship with the then-presidential candidate. To this point, the story is seedy but almost certainly legal.
The lies and omissions started almost immediately. As Rudolph Giuliani, the President's new lawyer, acknowledged Wednesday on Fox News, Daniels was paid because disclosure of her story on the eve of the election would have hurt Trump's chances. And who paid Daniels?
For months, Trump and his allies stonewalled -- that is, lied. Trump himself said he knew nothing about the payments,Sarah Huckabee Sanders, his press secretary -- presumably relying on the word of her boss -- said the same thing. Cohen himself put forward the implausible story that he paid the whole amount out of his own pocket as a reflection of his love for the Trump family.
Also on Fox Wednesday, Giuliani told an astonished Sean Hannity that Trump had indeed reimbursed Cohen for the payments to Daniels -- and he did so, according to Giuliani, in some peculiar and imprecise series of monthly payments.
Consider, alternatively, if Trump's team had told the truth from the start. He would have made a campaign report of a payment to Daniels, and that could have resulted in an embarrassing, but short-lived story.
Instead, the lies caused the Daniels fiasco to metastasize into a genuine crisis. Was there a campaign finance violation by Cohen -- or by Trump himself? Did Trump file a false financial disclosure report, or a false tax return, based on improper accounting for a gift? Did anyone (Trump or Cohen, for example) make a false statement to a bank in connection with this payment? And even now, do we really know who supplied the money to pay off Daniels?
Then of course there is the political (and moral) fallout of the falsehoods: why, now, should the public believe anything Trump says after he so obviously misled the public on this subject of wide public interest?
Trump's prevarications also made a mess of the even more consequential story of the firing of James Comey, the FBI director, in May of last year. The simple question of why Trump fired Comey has produced a still-changing collection of answers.
The White House originally said Comey was fired for mishandling the investigation of Hillary Clinton's emails. Then Trump himself told NBC's Lester Holt that the real reason was "the Russia thing" -- that is, to relieve the burden of the FBI's Russia investigation. (Trump apparently said a similar thing to a group of visiting Russian diplomats in the Oval Office.)
Wednesday, Giuliani offered still another explanation -- that Trump fired Comey because the director would not publicly clear the President in the Russia investigation.
Trump and his supporters have made the argument that the President had the right to fire Comey for any reason, or no reason at all. (This is legally questionable.) But surely settling on even a suspicious reason for firing Comey is better than the evolving story that the Trump camp has provided.
Multiple explanations succeed only in arousing suspicion -- which the President and his allies seem by now to richly deserve.