Maybe it's simple because you can do it. Can you take a temporal 3-D color scene and see it as a non-temporal 2-D B&W scene in your head?

Many Many people cannot do that. To some it's a total mystery.

If I run across the reference again, I'll post it but I don't have time to go thru my books looking for it. It's just an old quote that stuck with me.

-Mike