A government expert did not break the rules of evidence when he testified in a meth-trafficking trial that most couriers know they are transporting drugs, the Supreme Court held Thursday in a decision that produced dueling opinions from conservative Justices Clarence Thomas and Neil Gorsuch.

In its 6-3 ruling in Diaz v. U.S., the high court court rejected the argument that such testimony violated Federal Rule of Evidence 704(b), which prohibits experts from testifying about a defendant’s mental state when such knowledge is an element of the crime. Congress created Rule 704(b) following the high-profile trial of President Ronald Reagan’s shooter, John Hinckley, where experts offered competing opinions about Hinckley’s sanity.