I found this question on Quora.

Here’s my answer:

Most people experience a kind of image in their mind when they think of situations and things. It’s kind of like a dream, but unlike a dream there is no sense that the images are coming from the physical eyes.

The phrase “mind’s eye” refers to this experience by using the metaphor of an eye.

Some people do not have this experience, a condition known as aphantasia. Many of them report a sudden wash of realization when they understand that people who talk about seeing images in their heads are not speaking metaphorically.

The term aphantasia was popularized when Blake Ross published an article on Facebook about his experience: How It Feels To Be Blind In Your Mind.