Philosophers have long said that we experience the world through our senses, and scientists have confirmed this. The ability to see is one of our dominant senses. The question that Robert Henri is raising here is about our memory of what we see. Both painters and writers need strong visual memory in order to put detail into their work. Painters often work from observing models, but they also need to be able to work from the memory of that model. The same is true of writers. We must be able to describe our characters, the setting and the physical world. Often the physical world provides the reader with insight into the nature of a character.
Visual memory is something I struggle with both as a writer and a reader. When I come to a long descriptive passage in a novel, I will skim through it quickly so as not to be bored. When I write, I struggle to put in visual detail of the person and his surroundings. As a speaker I can be in a room for eight hours with a group of people, but at the end of the day I could not describe their faces or the clothes they were wearing.
The only place where I have discovered that I have a strong visual memory is when I am driving. I can have driven through a city once and come back five years later and I will remember visual elements and be able to find my way around without getting lost. Somehow subconsciously, my brain picks up the physical clues and I remember them when I am back in the same place, but if you were to ask me to describe the place I could not.
For the past fifteen years I have been cultivating my visual memory through the study of art. If your visual memory is weak like mine, I would encourage you to find ways to improve your visual literacy. Most creative leaders need a strong visual memory.