“It ain‘t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” - Mark Twain

In the mid-19th century, a physician named Dr. Ignaz Semmelweis was working as an assistant in Vienna General Hospital when he noticed something curious.

The hospital had two maternity clinics right next to each other, one staffed by doctors and medical students, the other by midwives.

Childbed fever—a bacterial infection impacting postpartum mothers—was rampant in hospitals of the era, but the two maternity clinics had dramatically different experiences with it.

The mortality rate of mothers at the clinic staffed by doctors and medical students was a shocking (even for the era) 18%, while it was just 2% at the clinic staffed by midwives.

As awareness of the difference spread, expectant mothers were literally begging to deliver at the midwife clinic.

Dr. Semmelweis scrutinized the differences in how the two clinics handled the births, but came up empty.

Then, a tragedy sparked an insight…

After a close friend in the hospital died of a similar infection following an accidental cut while performing an autopsy, it occurred to Semmelweis that the doctors and medical students were often moving straight from working on cadavers to delivering babies.

Without washing their hands in between.

He mandated handwashing with chlorinated lime prior to entering the maternity clinic.

And it worked: The mortality rate of the clinic staffed by the doctors and medical students immediately dropped from 18% to below 2%.

Unfortunately for Dr. Semmelweis, rather than being regarded as a hero, he was broadly ridiculed by the medical community.

Germ theory was non-existent at the time and many doctors resented the notion that their hands were the source of the clinic’s mortality problems.

Growing increasingly frustrated by the refusal of his peers to accept this new evidence, he was dismissed from the hospital. He spent years agitating for change, to no avail, and was eventually committed to an insane asylum, where he was beaten by guards and died of septic shock at just 47 years old.

The evidence had been overwhelming. The results had been undeniable.

And yet, none of that mattered.

The Semmelweis Reflex is the aptly-named human tendency to reject new evidence or knowledge because it contradicts one’s established beliefs or norms.

Why does this happen? Why do smart, accomplished people reject the very evidence that could help them?

There’s an old Zen parable that offers an answer:

A professor came to visit an old Zen master to seek advice.

"I have come to ask you to teach me about Zen," the professor said.

As the master began to teach, the professor repeatedly interrupted him to share his own opinions, knowledge, and stories.

The master suggested that the two should take a break to have tea.

The master poured the professor a cup. Once it was filled, he kept pouring, until the cup was overflowing hot tea all over the table, onto the floor, and onto the professor's robes.

"Stop!" cried the professor, "Can't you see the cup is already full?""

Precisely," the Zen master replied, "You are like this cup, so full of ideas that nothing more will fit in. Come back to me with an empty cup."

The doctors who ridiculed Semmelweis weren't stupid. They were accomplished. They were experts. That was precisely their problem. Accepting his evidence didn't just mean changing a procedure, it meant admitting the flaws in their deeply-entrenched beliefs.

There was no room in their cup for that.

The more you know, the fuller your cup becomes. All of your accumulated experience, recognition, and success cements a set of beliefs on the way the world works.

The full cup can’t accept anything new. There’s no room for new ideas. No ability to adapt. No space for uncertainty.

The antidote is what I call the Empty Cup Mindset:

An openness to new ideas, new information, and new perspectives—even when they challenge what you already know. Especially when they challenge what you already know. It’s a willingness to hold strong opinions loosely. To change your mind when the evidence demands it. To treat certainty as a warning sign, not a strength.

I’m increasingly convinced that the willingness to change your mind is the ultimate sign of intelligence. The most impressive people I know change their minds often in response to new information. It’s like a software update.

Author George Saunders put it beautifully:

“In a world full of people who seem to know everything, passionately, based on little (often slanted) information, where certainty is often mistaken for power, what a relief it is to be in the company of someone confident enough to stay unsure.”

To embrace an Empty Cup Mindset, you should regularly audit the flexibility of your beliefs and understanding of the world. Ask yourself this question:

What new evidence would cause me to change my mind on this topic?

If the answer is nothing (or close to it), your cup is too full.

The Empty Cup Mindset is a reminder that some of what you know today may be challenged by what you learn tomorrow.

And remember, that’s a good thing.

Because the goal isn't to be right—it’s to find the truth.