We generally live in a three-dimensional world. In some areas, that is reduced to two. For instance, printed material is on a flat surface, yet from that flat surface, we are able, through our imagination, to conjure up images that words in a book are telling us. We mentally move between two and three dimensions. When we see a movie on a flat screen, whilst it is two dimensions, we can imagine it in three dimensions; and with 3D glasses, we create the impression of three dimensions. So, it is with photos and many other things where, without thinking, we move between the two.