Imagine a horse. You can probably picture one in your head.
But if you're one of the 2% of people who live with "aphantasia" then you'll see nothing.
You know what a horse is, you know it has got four legs, but you just can't visualise one.
Aphantasia is a phenomenon which has only recently been defined by scientists.
Many people with it don't have a clue that they are different to anyone else.
When we discussed it on the Jeremy Vine Show on BBC Radio 2 a number of listeners came forward to tell us that we were describing their experience of life; that they had aphantasia.
Adam Zeman, a professor of cognitive and behavioural neurology, coined the term aphantasia with his team at the University of Exeter.
He said: "It's a lack of the mind's eye. An inability to visualise. It's an intriguing variation in experience, rather than a problematic condition; it's not a disease."
Aphantasia can have an impact on memory, because people are unable to visually recall moments in their life.
Prof Zeman says that as many as one in 50 people have aphantasia.
He says people either tend to have it or not - rather than experiencing it to different degrees of severity: "There is quite a big spread in the range of how well people visualise things.
"But anyone with aphantasia is way off the scale. So, in broad terms, most people without aphantasia can visualise quite vividly. But anyone who does have it cannot."
Some Radio 2 listeners were astonished to find their experience of life being described on national radio:
Rosie Edge, 63, Broadstairs:
"Listening to the programme, I found myself hearing someone describe exactly what I experience.
"My husband's always told me it was weird. Until you actually verbalise it, you don't realise it's different. I have two grown-up sons. If I think of them I don't get a picture in my mind.
"The first time I ever thought about it was when I had a conversation with my husband when I was in my 50s.
"We were talking about relaxation techniques and he said, 'Picture being by the sea in Cornwall.'" I said, 'Well, I can describe it in words.' And he said, 'No - actually visualise it. You must have an image of it.'"
"But I didn't. It was a confusing conversation; we couldn't even really work out if we were discussing the same thing. He described the image he had in his mind and that was completely alien to me.
"I have really poor recall of really important events in my life. I can't picture my wedding day but I can remember the emotional feeling."
Paul Arvidson, 47, Taunton:
"When I heard you describe this condition on the radio I nearly drove my van into a hedge. It's the first time I'd ever actually heard it described.
"I used to be a theatrical lighting designer. It's not that I don't know what things are, I just can't see them in my head. So I can perfectly well design the lighting for a show but I just need a lot of paper or a computer because I've got to physically write it all down.
"If somebody wants me to do some DIY and they describe what they're imagining, I can't do it without drawing it - or getting them to draw it.
"About seven years ago I wrote a book called The Dark which is a science fiction story set on an alien planet entirely without light.
"But it wasn't until I heard the programme that I realised… maybe that's what the book is all about! Perhaps the entire purpose of the book was a subconscious look inside my head."
Steve King, 44, Sheffield:
"I switched on the radio and I heard this - it was like a lightbulb moment.
"I'm 44 and I can't really remember anything of real significance. I'm almost embarrassed to say it but… my wedding day, the birth of my sons, birthdays, holidays, my honeymoon - anything big like that, I can't remember.
"I can't see any of it in my mind. My wife doesn't even bother to ask me about those events any more.
"When I look at photos of these events, it just reinforces the fact that I was there.
"People laugh at me and just think I have a really bad memory."
Dr Sarah Jarvis, a GP and BBC Radio 2's medical expert says: "The first academic paper which suggested this condition might exist was as early as 1880.
"It's very interesting that people who have aphantasia cannot visualise anything when their eyes are closed and they are awake, but they can have the same vivid dreams as everyone else.
"In can cause some difficulties in education because these days so much learning is done visually - right up to university level. Mind maps are very common. But people who have aphantasia simply can't picture the mind maps.
"We're only very recently beginning to recognise the impact this condition can have."