For the past few thousand years or so, the philosophy of space has occupied a lot of time among philosophers. There is really no argument as to the traditional definition of "space" as it usually relates to volume, being length width and breadth. But the big debate has typically been over the concept of empty space. That is, space that exists but does not contain any matter. On one hand, you had folks like the atomists arguing that empty space does exist, and has independent existence of matter contained in it. On the other hand, there were the philosophers who argued that space was merely a property of physical objects, and so there could be no space without matter. This debate really went unsettled until the past 80 years or so with the findings of modern science. Descartes and friends would be happy with the results, because modern physics suggests that there can be no space where there is no "thing" present, or more precisely a field. But one interesting question is how the debate ever got started in the first place. In other words, what causes one to think about the notion of "empty" and "filled" space in the first place? This may sound more like a question of psychology, but it has importance in the philosophy of space as well. Here's what I'm thinking. If we start with our everyday experience of physical objects, we can see that space does appear to be a property of things. Actually, we never actually *see* volumes, and no known human being has experienced anything in 3 dimensions. We only ever experience a 2D image of the outside world, with 2D geometrics from the retina being processed in certain areas of the visual cortex. From this 2D slice of space we form the concept of different objects through the different colors. Different areas of the plane we experience have different colors, and so we can make out the difference between a tree and the sky. And so we come to relate "substances" or "objects" with color. Any time we imagine a physical object, it necessarily has a color. There is no concept of an empty area, because there is a color at each point in our viewing space. But area is not the whole story, because we also have the concept of volume. Even though we only experience 2 dimensions, visual cues such as the differing sizes of objects, shadows and motion gives us depth perception. Through this we have the concept of the 3rd dimension, and objects with volume. But because we are able to view objects at differing distances, we understand that there must be invisible space in between us and such objects. IOW, we imagine that in the outside world, there are large volumes of space which cannot be seen, and that in turn leads us to think there are no objects present in this distance. Of course, if our brains are processing the data from light hitting our retinas, then there must necessarily be regions of space which are not emitting light. So invisible space must be a fact. What I'm thinking is that humans have taken depth perception and mistakenly believed invisible and visible space to be fundementally ontologically different. It seems silly that we would do such a thing on the basis of mere vision. It would be like calling AIR nothingness or a void because we can't see it. This intuition seems to be hard to shake, because imagining empty space as just another thing is difficult. In order to think of some geometric object, we must give it a color, and giving empty space any kind of color (so as to imagine it as a substance) feels unnatural. We can only imagine an empty volume through the relations of 2 dimensional objects, so I can at least understand why inution alone would lead us to the idea of some ontologically different and "empty" kind of space. But this would be a case of intuition alone leading to horrible reasoning. So without our perception of depth, what would happen to the philosophy of space? What if we lived in a world where the air glowed bright, so as to prevent our ability to perceive depth? We would still see motion of objects through space, but only as other objects (colors) were displaced. I'm wondering if there would have ever been a debate between proponents of relationalism and substantialism. There might be no atomists void, no debates between Newton and Leibniz, and no theologians loosing sleep over the issue. Well, it's an idea anyway. Though it would seem silly that such a hotly debated issue with a long history could come from a simple trick of the brain.