Justice Elena Kagan should recuse herself from Suncor Energy v. Boulder County. Not because a lower court told her to. Not because an ethics board ruled against her. Because the oldest test in the profession says so: would a reasonable, informed observer question her impartiality? Here, the answer isn’t close.

I spend a good part of my career serving as an expert witness in securities and fiduciary litigation. Before I say a word to counsel, I clear conflicts, in writing, every time. Not because I’m worried about actual bias. Because the standard doesn’t ask whether I feel biased. It asks whether a reasonable person, looking at my history with the parties or the subject matter, would wonder. If the answer is yes, I will step aside. No hearing required. No committee vote. That’s the discipline the profession imposes on people far less consequential than a sitting justice of the Supreme Court.