um... actually, we live in a 4-D world. ''3-D'' engines are somewhat ''4-D'', ''2-D'' games even have this dimension.
Time. we perceive height, width, depth, and time.
now, no game (except maybe MMORPGs) truely shows time passage as you can go back at anytime and do it again. it might show how you killed a room of badies an hour ago. some older games, if you go back to a previous room, the badies are all back, that doesnt show time passage at all.
anybody see memento? that is someone who has almost no concept of time, and thus the 4th dimension.
I hurt my eye as a little kid and can see now out of only one eye. this limits my depth perception, and thus i percieve the 3rd dimension alot less than you all do. (actually why im interested in 3d games)
and i will agree that 3d engines are really 3d-2d engines, however in modern days i believe the graphics card or something *not written or messed with by the programmer* actually makes this conversion, thus the actual engine is still in 3-d, from front to back. now, it may keep in mind that it has to convert to 2-d later on and so does some odd routines in areas because of this.