Walter Thornhill recently said that when it comes to physics, we can only perceive and work with three dimensions. The fourth dimension, time, is something beyond our perception and there is no "fabric" of space and time. However, when it comes to Photoshop and it's ilk of digital graphics programs (save Maya, 3DS, DAZ Studio, Vue, etc.) you work with two dimensions.
Scanning a photograph and digitizing it means you are taking it from 3 dimensions into two. Playing around with it in photoshop means that you are working in two dimensions. A two dimensional object lacks the third dimension, which gives it depth and solidity. With out it, it's perfectly flat, two dimensions. Something that only exists in 2 dimensions only exists in photoshop.
It's simple to get, and it's simple to understand.