There are two kinds of existence, subjective and objective. Since it's all we know directly, we tend to try to apply our reasoning about subjective things to objective things. For example, it seems that matter is real where as energy is only a mathematical abstraction. But theorerically, they are on equal footing. It's just that we interact directly with matter every day, and thus we have a more concrete mental representation of it. This mental representation is not matter, and while in our mental model, matter is certainly more real than energy, we don't know that to be the case in the objective world. All we know about the objective world, our ideas of space, time, mass, etc, is just our mental model of it. I'm not saying the physical world isn't real, but maybe it's just a different kind of real than our mental world. Is this possible, for there to be two different kinds of real? Taking the subjective as real, there is no way to show the objective, physical world exists. It could all be in our head. Taking the physical world as real, theres no way to demonstrate the existence of conscious subjectiveness at all. However, they do seem to affect each other. There is no doubt the physical affects the subjective, if the physical really exists, and since these words about subjectivity are in a computers memory right now, the subjective must be affecting the physical if it exists. So there is definitely a distinction between the two, but also some kind of interaction. There is also a symmetry between them. The physical world may exist, and if so, it affects the mental world, but this cannot be proven from within the mental world. Exchange the words physical and mental in that sentence and it's still true. Is there any significance to this symmetry? Is there some base that contains both the mental and physical, or are they separate universes completely? Is real a quality that, if true in one world, must necessarily be true in the other?