"Goodbye world," wrote Brittany Maynard, a 29-year-old woman suffering from an aggressive and lethal form of brain cancer, before she took her own life on Saturday. "Spread good energy. Pay it forward!"

Maynard captivated online audiences and reignited the right-to-die debate after she announced in a viral video that she would take her own life rather than die the painful death her brain tumor was predicted to soon cause.

In January, she was diagnosed with glioblastoma multiforme. In April, she was told she had six months to live. Few patients with her condition live longer than three years, regardless of treatment. She and her husband moved from California to Oregon, one of five states with so-called "aid-in-dying" laws. Last month, Maynard and her family visited the Grand Canyon. She announced she would take her life on November 1, and she did.

One of the most fascinating aspects of Maynard's story is how clear-eyed and peaceful she seemed in photos and videos she released in the months leading up to her death. Her contented affect might be one reason why National Right to Life, which opposes physician-assisted suicide, called her a woman "in the prime of her life," even though, suffering from crippling seizures and stroke-like symptoms, she was clearly far from her "prime."

And that raises an interesting catch-22 when it comes to death-with-dignity laws: Maynard might not have seemed, to some, like she was ready to die, because most terminally ill people who seek assisted suicide are depressed. But depression also makes physicians less likely to prescribe the lethal medications that would allow terminally ill patients to die by their own hand.

Oregon has the longest-standing aid-in-dying law in the U.S., but since it was enacted in 1997, just 752 people have taken their own lives under the law, or .2 percent of all deaths in the state during that time, as Vox points out. For a patient to receive the lethal medication in Oregon, he or she must have a terminal diagnosis, confirmed by two doctors, and have only six months to live. The patient can't have an underlying mental-health issue, and his or her request must be verified by two witnesses. Patients must make one request for the medications in writing and one orally, and there's a 15-day waiting period in between the two requests.

Oregon physicians reject five out of six requests for the lethal medication.

One reason: Physician-assisted suicide requests are less likely to be honored if the patient sees themselves as a burden or if they're depressed. But because feeling unwanted and suicidal ideation can be two symptoms of depression, physicians may have difficulty knowing whether the patient would have a change of heart if their mental state improved. Does the patient want to die because they're depressed, or because they're terminally ill—or some combination?

In a study that compared 55 Oregonians who had requested physician-assisted suicide with 39 terminally ill people who did not request the medications, those seeking suicide were more likely to be depressed, hopeless, not spiritual, and self-reliant. As the bioethicist Ezekiel Emanuel wrote in The Atlantic in 1997, patients who are depressed are likely to seek physician-assisted suicide; patients who are in pain are not.

"More than a third of the patients requested assistance with suicide because they perceived themselves as a burden to others, but only three of these patients received prescriptions for lethal medications," a 2000 study of the Oregon law found, "suggesting that the physicians were reluctant to accede to requests for assistance under these circumstances."

The doctors' selectiveness is merited: 11 percent of the patients who were either medicated for depression or evaluated by a mental health expert changed their minds about wanting to kill themselves. That's a small percentage, in terms of a clinical study. It's a large percentage, in terms of an irreversible, life-or-death decision.

Then again, while depression can be treated, things like a lack of spirituality or a self-reliant lifestyle can't exactly be medicated away.

The debate over whether mental illness can complicate a patient's wish to die is already very real in other parts of the world.

In the Netherlands, voluntary euthanasia has been legal since 2001, and the country has recently made forays into offering physician-assisted suicide to psychiatric patients, rather than just to terminally ill ones. In 2013, the psychiatrist Gerty Casteelen helped kill a healthy 63-year-old man who was dreading his retirement.

Meanwhile, Switzerland, as my colleague Julie Beck wrote, has few regulations when it comes to voluntary euthanasia, and the country attracts hundreds of so-called "suicide tourists" every year.

The U.S. is far behind Europe when it comes to assisted-suicide laws, and Oregon's experience shows that doctors can often catch symptoms of depression in voluntary-suicide patients. But even in states that don't have aid-in-dying laws, physicians sometimes perform euthanasia through other, more discreet means, like sedating patients heavily or helping them starve to death.

There are a number of questions prompted by Maynard's death, but one of the most troubling is, what happens when the patient seeking lethal medications isn't as bright, purposeful, and tranquil as Maynard was? How do we know if someone, besides being ravaged by their body, is also tormented by their mind? And should it matter?