In his book "The Human Story," evolutionary psychologist Robin Dunbar speculates about what makes us human. He writes, "What sets us apart is, above all, a life in the mind, the ability to imagine."

But what if you can't imagine?

Many of us take for granted that we can envision the faces of our loved ones simply by thinking of them, picture the events of a novel in our mind's eye as we read or count sheep to fall asleep. But not everyone can do this, which is a relatively new discovery. In fact, the condition only received a name in 2015: aphantasia, which essentially means "the absence of fantasy."

People with aphantasia can't conjure mental images, and while most suspect that there's something different about them, others may go their entire lives without realizing they're experiencing the world differently.

Software engineer Blake Ross only recently discovered he has aphantasia, which finally gave a name to the "lifelong otherness" he'd experienced.

"I know nobody can see fantastical images through their actual eyes," he wrote in a personal essay. "But I was equally sure nobody could 'see' them with some fanciful 'mind's eye,' either. That was just a colorful figure of speech, like 'the bee's knees' or 'the cat's pajamas.' I thought 'counting sheep' was a metaphor."

What causes aphantasia?

Several regions of the brain must work together to generate images based on memories, so scientists suspect people who are unable to visualize must have a disruption in some part of this neural network. Some people are born with the condition, but others may develop aphantasia as a result of a mood disorder or brain injury.

A 65-year-old retired man first brought the condition to neurologist Adam Zeman's attention in 2005. The man, identified in papers as simply MX, came to Zeman after a minor medical procedure rendered him unable to conjure images in his mind.

Zeman and his team scanned MX's brain as he looked at faces of famous people and named them, and they found the correct regions of his brain became active throughout this task. However, when they showed people's names to MX and asked him to picture those people's faces, none of those facial-recognition brain regions became active like they normally would.

Unexpectedly though, MX was able to recall the color of people's eyes when asked, as well as tell researchers which letters of the alphabet have tails, such as "g" and "j," suggesting his brain was using a different strategy to solve visual problems.

Zeman has since discovered more people who lack this mind's eye, and testing shows their brains work very similarly to MX's. While some of them have reported seeing brief flashes of imagery when asked to picture something, most are unable to visualize anything at all. However, when asked to recall the number of windows in their home, most are able to do so even though they can't picture their home. Their brains have also found a way to solve visual problems without the ability to visualize.

How common is this condition? Scientists aren't sure, but they suspect it affects only 2 to 3% of the population — and many of them are in creative fields where you would think the ability to visualize would be paramount.

Up to 3% of the population may not be able to conjure mental images. Ollyy/Shutterstock

Ed Catmull, former president of Pixar and Walt Disney Animation Studios, is one of them. As the BBC explains, Catmull revolutionized 3D graphics, and the method he developed for animating curved surfaces became the industry standard.

Oscar-winner Glen Keane, who created Ariel of "The Little Mermaid," also has no visual imagery — yet there’s no question of his creative ability.

"He is truly extraordinary, he's one of the best animators in the history of hand-drawn animation," Catmull told the BBC. "[And] he said that he could never visualise either."

What's life like without visualization?

People with aphantasia say they've often felt isolated from others. Jaromir Chalabala/Shutterstock

Although it appears people with aphantasia have ways to work around their lack of visualization, the inability to summon images can have a dramatic effect on their lives. Often, they report feeling "isolated" after learning people can see things they can't. They say it's upsetting to be unable to picture loved ones. And, because our memories are so tied to our visual experiences, being unable to conjure images can affect memory.

People with aphantasia often struggle to recognize faces, a condition known as prosopagnosia, and while they may be skilled at recalling facts, they are unlikely to remember directions or how their spouse looked on their wedding day.

Ross says even telling someone how his day was can be difficult because he can't visualize it.

"It is hard not to feel like a sociopath when you're lying about how you spent your Monday and you don't even know why. And there is a sadness, an unflagging detachment that comes from forgetting your own existence. My college girlfriend passed away. Now I cannot 'see' So-Youn's face or any of the times we shared together."

And while he's not sure if it's the lack of visualization to blame or not, Ross says the realization that he has aphantasia has alerted him to other experiences he's missing out on.

"I've never had a song 'stuck' in my head," he writes. "I have no sensory experience in my mind of any discernible nature. Thinking about a beach doesn't make me feel calm; thinking about a tarantula doesn't give me goosebumps. I can't 'recall' the taste of pizza, the feel of Velcro, or the smell of Ghirardelli Square."

However, Zeman says just because someone can't visualize things in their mind's eye doesn't mean they can't live satisfying lives. "Lives without imagery can be rich and fulfilling," he writes. "But they are different, and remind us of the great but easily unnoticed range of variation in human experience."

People with synesthesia and ASMR, for example — and even those who speak various languages — all experience the world in a different way, but often we simply assume everyone is experiencing it the same way we are. As neuroscientist David Eagleman puts it, "We all accept the reality presented to us."

Think you may have aphantasia? Take the survey created by the University of Exeter, where you can also find contact information if you have more questions.