But the choice not to know can also be liberating. You can say, “No thanks, I opt out.” And there’s actually a tool to help you do that. It was developed by Steve Scheier, an expert in organizational decision making, in honor of his wife, who died of bile duct cancer in 2010.

Just as clinicians have pain charts — a series of frowny and smiley faces — that help determine how severe patients’ discomfort is, Mr. Scheier invented a chart to help patients think through what they wanted to know about their illness. “You’re filling out 40 other forms, for god’s sake,” he said. “This is an opportunity for you to advocate for yourself and tell your physicians, so they know what to do.”

He calls the form a Prognosis Declaration, and it allows patients to choose among a few options:

Tell me everything.

I’ve not decided what I want to know about my prognosis, so ask me over the course of my treatment.

I want to participate in my treatment, but I don’t want to receive any information on my prognosis.

I don’t wish to know any information about my prognosis but I authorize you to speak with [blank] about my case and for you to answer any questions that this person may have about my likely prognosis and treatment.

Mr. Scheier’s wife, Amy, was 50 when she was told she had cancer. She learned that the five-year survival rate was zero, and if left untreated, less than a year. She didn’t want to know any more. So while she jumped into treatment and took all the chemo her doctor ordered, she kept working, seeing friends and traveling, choosing not to pay attention to her prognosis.

On what would turn out to be their last visit to her oncologist, Mr. Scheier asked his wife if she’d changed her mind and now wanted to know how much time the doctor thought she had left. She said yes. The answer was a couple of months. She died the next week.

It’s not clear whether the Prognosis Declaration will ever find its way into standard medical practice. But the fact is it belongs in the hands of patients rather than doctors, anyway. It’s a springboard to help people think through how they want to be cared for when they’re in a vulnerable state and need to make decisions.

Choosing not to know some of the details of your condition is a rare choice, but having a choice at all is recent in the history of medicine. Until the 1980s, “doctor knows best” paternalism relieved physicians of any obligation to share a prognosis — or even a diagnosis — with patients. In a 1961 survey, 90 percent of doctors said they preferred to not tell a patient he or she had cancer. Many people died without even knowing what killed them.

That may offend contemporary ears, but it’s not necessarily a story of negligence or dishonesty, nor is the idea of a Prognosis Declaration a vote for denial. It’s simply an acknowledgment that knowing more sometimes serves us less . And no matter what we choose, we never really get to know everything.

BJ Miller, a hospice and palliative medicine doctor at the University of California, San Francisco, and Shoshana Berger, a writer and editorial director at Ideo, are the authors of the forthcoming book “A Beginner’s Guide to the End: Practical Advice for Living Life and Facing Death.”