perspective

From Wikipedia, perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects.

As objects become more distant they appear smaller because their visual angle decreases. The visual angle of an object is the angle subtended at the eye by a triangle with the object at its base. The greater the distance of the object from the eye, the greater is the height of this triangle, and the less the visual angle. This follows simply from Euclidean geometry.

http://en.wikipedia.org/wiki/Perspective_(visual)

In our daily life, we believe that perspective is the only true way to see. However, after studying its history, I found that perspective has been subjected to intense and often hostile interrogation by philosophers and critics as well as artists in the early days.  Also, the perspective picture is not an real image of what we see, unless we view the picture with one eye from a fixed point that corresponds exactly with the centre of projecton assumed by the artist when he made the picture.