Whether prosecutors can view what may be the key clue to the planning and motives behind the Aurora theater massacre might hinge on an obscure point of law: Is a psychiatrist a physician or a psychologist?

Arapahoe County prosecutors and attorneys for shooting suspect James Holmes will be back in court Thursday to debate that question in what will probably be the biggest legal fight yet in the case.

The two sides are expected to argue over whether prosecutors can look at a package Holmes mailed to his psychiatrist at the University of Colorado just before the shooting. Holmes’ attorneys have asserted that the notebook is protected by doctor-patient privilege.

Holmes has been charged with killing 12 and injuring 58 on July 20 in the Century Aurora 16 during the premiere of the new Batman movie, “The Dark Knight Rises.”

The package, which reportedly contains a notebook, could prove to be the case’s Rosetta stone, explaining unanswered questions of how and why the attack occurred.

In a hearing last Thursday, both sides hinted at how important the legal standing of psychiatrists would be to the debate.

Prosecutor Karen Pearson argued that Holmes’ psychiatrist, Dr. Lynne Fenton, should be considered a physician under the law, thus creating a lower protection of confidentiality.

“Statements to a psychiatrist are not ipso facto privileged,” Pearson said.

Holmes’ attorney Daniel King said Fenton was providing mental-health services and, thus, was more like a psychologist. King said Pearson’s argument “flies in the face of reason” because it would mean that a psychiatrist — who is a licensed medical doctor — would be entitled to less confidentially than a psychologist.

The distinction is important because Colorado law has different definitions for confidentiality with physicians and psychologists but makes no mention of psychiatrists.

For physicians, the law protects “any information acquired in attending the patient that was necessary to enable him or her to prescribe or act for the patient.”

For psychologists — as well as professional counselors, therapists, social workers and registered psychotherapists — the law protects “any communication made by the client to the licensee, certificate holder, or registrant or the licensee’s, certificate holder’s, or registrant’s advice given in the course of professional employment.”

Prosecutors are expected to argue at Thursday’s hearing that Holmes, who withdrew from the university prior to the shootings, had ceased his doctor-patient relationship with Fenton. That might mean — if Fenton is viewed as a physician under the law — that the notebook was not part of Holmes’ treatment.

Denver attorney Karen Steinhauser, a former prosecutor, said she knows of no cases that define whether a psychiatrist is a physician or a psychologist in circumstances like this. But, she said, it might not matter.

If Holmes mailed the package as part of a plea for Fenton to help him, Steinhauser said, it would still be protected, even if Fenton is viewed as a physician.

“I’m not sure one provides any greater protection in terms of the privilege than another,” she said.

The judge in the case, 18th Judicial District Chief Judge William Sylvester, gave prosecutors a partial victory on an underlying issue earlier this week when he ruled prosecutors could see some of Holmes’ educational records.

Holmes is charged with 24 counts of murder and 116 counts of attempted murder in connection with the shooting.

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold