Donald Trump has a lot of experience with lawyers. He has employed fleets of counselors as a businessman and now as a politician. Still, the client in chief is unaware of a key legal principle that seems increasingly relevant to his predicament.

Trump’s former personal attorney, Michael Cohen, was sentenced to three years in prison today (Dec. 12) for federal crimes including tax evasion, campaign-finance violations, and lying to Congress and the IRS. He struck a plea deal with federal prosecutors in the Southern District of New York last week.

Cohen says that Trump, while running for office, instructed him to pay off two women in exchange for silence about their alleged affairs with him. “Time and time again I felt it was my duty to cover up [Trump’s] dirty deeds,” Cohen said at his sentencing hearing.

The president, for his part, blames Cohen, for any possible violations of the law. “Michael Cohen is a lawyer,” Trump told Reuters yesterday (Dec. 11). “I assume he would know what he’s doing.”

However, as the great American jurist Oliver Wendell Holmes put it in his seminal 1881 legal text, The Common Law, “Ignorance of the law is no excuse for breaking it.”

Ignorantia legis neminem excusat is a classic legal principle (pdf), dating back millennia to Roman civilization. It’s Latin for “ignorance of the law excuses no one” and means just what it says. Not knowing you’re breaking a law is not a viable defense if you’re charged with a violation. You’re responsible for following the laws, whether or not you can articulate them yourself, and can be held liable for violating strictures of which you were unaware.

If the president is charged and it’s proven that he did indeed direct Cohen’s actions, pleading reliance on his attorney’s judgment and arguing that he was unaware he was breaking rules will not help him evade liability. It’s an important notion many of us don’t know but all of us should keep in mind, especially Donald Trump,

He may no longer be able to afford to ignore his own ignorance.