"To see is to think"
Salvador Dalí

TALKING ABOUT THE NATURE OF PERCEPTION in general and especially visual perception, a simple notion would be that to perceive is simply to mirror the objects in the world such that the physical properties of these objects are reflected in the mind.
But is it really the case that we see in that way as if we only measure the scene we watch? The following examples will show us that perception is different from this simple notion and more constructive.

The central circle surrounded by the small circles look larger than the central circle surrounded by the large circles. But physically, there are both exactly the same size. 

This is called the Ebbinghaus illusion or the Titchener illusion and is only one of many examples where the physical size does not correspond to the perceived size. 

On the left you see first a dot moving from the left to the right along a straight line and then a dot moving in the same direction on a curved path. But when both moving dots are presented at the same time,you do not see simply two dots each moving in the described way.
More likely you perceive one dot rotating around the other and both together moving to the right. In this example the visual system creates an interpretation which goes beyond the parts of which the scene is composed of. 

In the figure on the left you see two alternating frames. Note that the central rectangle within the dark surround looks brighter than the rectangle within the bright surround. But in both frames the central rectangles have exactly the same intensity. 

In this example the perceived brightness does not simply correspond to the physical intensity and is influenced by the context.

These examples (and many other demonstrations) support the conclusion that perception does not merely reflect the world. Perceived size is not the same as physical size(Fig.1), perceived brightness is not the same than physical intensity (Fig.3), perceived velocity is not physical velocity and so on for many other perceptual attributes. Moreover, the perception of composite stimuli often elicits interpretations which are not present when the components are perceived separately (Fig.2). Or to put it in other words: 'The whole is different from the sum of its parts." The Gestalt Laws deals with this aspect in greater detail.

The nature of perception is rather to provide a useful description of objects in the outside world instead of being an accurate mirror image of the physical world. This description has to represent features that are relevant to our behavior.

by Mónica González-Pérez ，Dept. of Educational Technology ，San Diego State University ，Original drawings by Jürgen Golz