Look into a bright light bulb or a camera flash, then look away and examine the after-image. What is it exactly that you are looking at? It is a patch of color due to saturation of the retina by the bright light. But think about it — are you looking at your retina when you view that colored patch? No, because my retina is up there in the back of my eyeball, but this image is out there in space before me, although it jumps to wherever I turn my gaze, as if it were being projected out of my retina into empty space. Is that what is going on? Well, to trace the causal chain of vision, light from the world enters your eye, where it is transduced to an electrochemical signal which is transmitted up the optic nerve to your brain. The colored patch that you see is the effect of the retinal saturation on the image sent from the eye to the brain. But my brain is back here inside my skull, and that image is hanging in space out in front of my eyes!
Consider another peculiarity of vision: Close your eyes and see what happens. The world you saw around you suddenly disappears, as if your eyelids were blocking the view from your eyeballs so all you can see is the inside of the closed lids. Is that what is really happening? Lets think about it: The world around you can be factored into objective and subjective components. There is the world itself that exists independent of my experience of it, and then there is my experience of the world as I see it with eyes open. When I close my eyes, the objective world continues to exist uninterrupted, but my subjective experience of the world has blinked out of existence due to the blockage of my view by my eyelids. So the colored three-dimensional world that blinks out of existence when we lower our lids, whose existence is causally dependent on the state of my eyelids, that is the subjective world of my experience, a view of the world from my perspective. How can three-dimensional colored moving images exist in the seething grey matter of my brain? Or are the images of our experience projected out of our head to appear superimposed on the world around us?
There is a third peculiarity of vision that you can see if you stand in a long hallway. Does the far end of the hallway look smaller than the nearer portions? Do the sides of the hallway seem to converge in the distance? Do the sides of the hallway actually converge into the distance? If not, then why do they appear as if they do? And when you stand on a long straight road or a railway track, the sides of the road appear to converge to a point on the horizon.
Subjects in a hallway are presented with three cardboard models, and asked which model most resembled their experience of the hallway. Most subjects picked model B.
They were then offered a third model with grid lines overlaid, and they were told that this is a scale model, but that the scale varies with depth into the model. In other words this model embodies the same duality in size perception plainly evident in the hallway itself: Things in the distance appear smaller by perspective, but at the same time they appear undiminished in size!
When offered this alternative, 90% of the subjects chose this model as the best representation of their experience of the hallway.
This experiment reveals that our experience has a variable representational scale, in other words, the spatial scale of our experience gets smaller and smaller into the distance, reaching perceptual infinity at the dome of the sky, where the distant stars appear as if on a spherical surface.
Nowhere in the objective world of external reality is there anything remotely resembling perspective as we observe it in phenomenal experience. The prominant violation of Euclidean geometry in phenomenal perspective is perhaps the clearest evidence for the world of experience as an internal rather than an external entity, for the curvature of perceived space is clearly not a property of the world itself, only of our perceptual representation of it.
The Curvature of Perceived Space
What does it mean for a space to be curved? If it is the space itself which is curved, rather than just the objects within that space, then it is the definition of straightness itself which is curved in that space. In other words if the space were filled with a set of grid-lines marking straight lines with uniform spacing, those lines themselves would be curved rather than straight, as they are in Euclidean space. However the curvature would not be apparent to creatures who live in that curved space, because the curves that are followed by those grid lines are the very definition of straightess in that space. In other words a curved object in that curved space would be defined as perfectly straight, as long as the curvature of the object exactly matched the curvature of the space it was in. If you are having difficulty picturing this paradoxical concept, and suspect that it embodies a contradiction in terms, just look at phenomenal perspective which has exactly that paradoxical property. For phenomenal perspective embodies exactly that same contradiction in terms, with parallel lines meeting at two points while passing to either side of the percipient, and while being perceived to be straight and parallel and equidistant throughout their length. This absurd contradiction is clearly not a property of the physical world, which is measurably Euclidean at least at the familiar scale of our everyday environment. Therefore that curvature must be a property of perceived space, thereby confirming that perceived space is not the same as the external space of which it is an imperfect replica.
In fact, the observed warping of perceived space is exactly the property that allows the finite representational space to encode an infinite external space. This property is achieved by using a variable representational scale, i.e. the ratio of the physical distance in the perceptual representation relative to the distance in external space that it represents. This scale is observed to vary as a function of distance from the center of our perceived world, such that objects close to the body are encoded at a larger representational scale than objects in the distance, and beyond a certain limiting distance the representational scale, at least in the depth dimension, falls to zero, i.e. objects beyond a certain distance lose all perceptual depth. This is seen for example where the sun and moon and distant mountains appear as if cut out of paper and pasted against the dome of the sky.
The world really is all in your head!