WASHINGTON—As the White House finished final vetting of Supreme Court nominee Neil Gorsuch in January, someone spotted a crucial omission in his dossier: the doctoral dissertation he had written at Oxford University constructing a legal, practical and moral case against assisted suicide.

The one person sure to have the manuscript was John Finnis, Judge Gorsuch’s Oxford adviser and a champion of the legal philosophy known as natural law. White House aides, determined to avoid last-minute surprises, tracked him down.

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