Gestalt Psychology

Gestalt psychology is a theory of mind and brain positing that the operational principle of the brain is holistic, parallel, and analog, with self-organizing tendencies, or that the whole is different from the sum of its parts. In short term, means you may see one thing at first then you might see something else later. Lets put it at an example: 

   What do you see here? Do you see a dog in a park or something else? When you see this picture it might take you a while to see the dog sniffing at the grown. This is called emergence. Other pictures will do this to you too. Such as this one...

This one is a reification piece it is a constructive or generative aspect of what you see

For instance, a triangle will be perceived in picture A, although no triangle has not actually been drawn. In pictures B and D the eye will recognize disparate shapes as "belonging" to a single shape, in C a complete three-dimensional shape is seen, where in actuality no such thing is drawn.

Reification can be explained by progress in the study of illusory contours, which are treated by the visual system as "real" contours.

These pieces are multistable perception, these tend to pop back and forth unstable between two or more alternative interpretations. This is seen for example in the Necker cube, and in Rubin's Figure / Vase illusion shown to the left. Other examples include the 'three-pronged widget' and artist M.C. Escher's artwork and the appearance of flashing marquee lights moving first one direction and then suddenly the other. Again, Gestalt does not explain how images appear multistable, only that they do.

Invariance is the property of perception whereby simple geometrical objects are recognized independent of rotation, translation, and scale; as well as several other variations such as elastic deformations, different lighting, and different component features. For example, the objects in A in the figure are all immediately recognized as the same basic shape, which are immediately distinguishable from the forms in B. They are even recognized despite perspective and elastic deformations as in C, and when depicted using different graphic elements as in D. Computational theories of vision, such as those by David Marr, have had more success in explaining how objects are classified.

Emergence, reification, multistability, and invariance are not separable modules to be modeled individually, but they are different aspects of a single unified dynamic mechanism.

In seeing these pictures you understand what gestalt psychology is.

http://en.wikipedia.org/wiki/Gestalt_psychology