Gestalt psychology...it's about the parts as a whole...

Upon the first look at the image on the left, most people see a silhouette type image of a woman. However, upon closer inspection and a bit of tricking of our eyes we can see that the darker image is a man playing a saxophone. 

The trick that our eyes play on us is part of Gestalt psychology.

 

This psychology consists of several laws:

Law of Closure: Just because an image isn't quite complete, like the one on the left, doesn't mean that our eyes not are able to "close" the image into something recognizable. For example, the facial shape of the woman on the left is created based on the positive and negative shapes that are present.

Law of Similarity: Basically, anything that is similar or that we know to be similar, our brains automatically try to group together. For instance, we know that humans have two eyes and the basic outline of one is created on the left of the above image and the basic shape of one is also created and together, the brain is able to, in a round-a-bout way, piece them together and make recognizable things out of them.

Law of Proximity: A good example of this would be placing dots on a piece of paper and being able to draw a line to connect them with our eyes when a line isn't actually present. Our mind tries to group things together and make sense of things that might randomly be spaced apart. In the case of the dots, a line could be created rather than a jumble of random markings.

Law of Symmetry (or figure ground relationships): The next image is a good example of this. There is obviously a window shade present,  but based on the spacing of the lines and also on the shapes of the lines, we are able to make out the shape of a face profile. 

Law of Continuity: We are able to recognize through the image below that the "line" created from the circles on the left "continues" through both the above grouping and below grouping of circles as well.

Law of Common Fate: When the viewer sees similar objects or images facing/pointing in the same direction, there is a common thing between them all. However, if one of the objects or images is placed differently then tension is created for the viewer and there is no commonality with the images.