David Hockney wrote a book ("Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters") on the contention that the "secret" of almost all artists from the Renaissance to the 19th century used the Camera Obscura, which essentially projects an image onto a screen just like a pin-hole camera does. The artist vastly simplifies his task by essentially traceing a reverse image onto the canvas or paper. So it is really a primitive form of photography.
Our eyes are constantly flitting about, taking in a vast amount of what might seem to be a confusing array of light and colour. Yet we experience it as an unbroken continuous 3-dimensional space around us. It is the brain that is adjusting and reversing the effects of they eye and head movements. The brain is constantly simplifying and abstracting external reality to something else. It translates them to symbols for one thing, it interprets 2 separate 2-D images into 3-dimensions, so that we actually feel experience our existence in a 3 dimensional realm. It maps the whole world onto a whole internal encyclopedia of our preconceived notions and experiences, biases, predudices. It can also open doors to new ideas and ways of thinking.
That is, the seeing is done by the brain, or maybe more acurately, by the mind! Seeing is tied in with our whole experience of life. It is no wonder art has so many branches, ideas and philosophies.
This is not to say all art is necessarily good or well-conceived. But be careful what judgments you make of a genre or an artist. Try to experience what they experience and think as you see a painting.