The absolute majority of beings (human or animal) have two eyes. They generate two plane images but what we see is a single tri-dimensional image (photographic-type image model) in accordance with MDT. Moreover, if we have a single plane image (we look with one eye) the brain will continue to build the tri-dimensional model. But we have got a problem: with a single plane image we have not enough information to build a tri-dimensional image. However we have a "compensation": the brain is an extremely powerful system. It will use any kind of supplementary information to build first a tri-dimensional image and then, the tri-dimensional model. In the following, we will describe an exercise for beginners to demonstrate this. We need to watch TV with a single eye in a room with no additional light source. The glass surface of the screen has to be absolutely invisible (there should be absolutely no reflection of light on it). We have to sit in front of a normal screen at least at 3 meters distance (we should not be able to see the pixels which build up the image). The screen should show a familiar picture, from common external reality, in normal perspective, and the image has to change slowly. If, under these circumstances, we watch the screen with one eye, after some training, we will see a tri-dimensional image. This experiment proves directly that the basic function of the brain is to make image models. The generation of tri-dimensional models by the brain starting from a plane image is known for a long time. This appeared at the same time with the expansion of art painting trade, many hundreds of years ago. Thus, a painter used to paint first the foreground, and later the background. A good painter had the whole tri-dimensional model in his head, and the background connected perfectly with the foreground, even if the background was painted a lot later. In some paintings, the background or some components of the painting do not match perfectly (a poor painter) and this could be noticed by art experts looking at the painting with one eye. Rembrandt painted scenes with groups of people. However, some people in the group could be "closer" or "farther" from the viewer. When such a compact group is watched with a single eye, one can notice that the painter had painted them correctly (the persons farther out are slightly smaller). Our brain can notice tiny differences, because it reconstructs the 3-D model. By the way: to build a 3D model based on a single plane image is an operation which requires an immense capacity of processing of information. In spite of its huge power, the brain has problems with the capacity of processing such a huge amount of information. As in principle there is not enough information for such an operation, the brain has to guess one or several probable models, which have to be verified. From my direct experience, in order to guess a 3D model from a plane image ones has to be in a very good physical and psychical shape.