Seeing with your brain
While the eyes are the most important instruments for recording the world around us, it is the brain that - lightning-fast - interprets all the information the eyes send to it, to give us a “clear” idea of the world as we see it. Actually, our brain compares all information it receives with the “model” it has stored in its memory from previous experiences. For instance, it calculates depth based on the knowledge that objects which appear smaller or out of focus in our vision are farther away. Together with factors such as lighting and shadow this is what our 3-dimensional view is based on.
If certain parts of information are missing these are filled in based on previous experiences. Therefore it is rather easy to lead our brain astray and make it believe something is real while in fact it isn't. Visit the Grand Illusions (click the above image) website for some fascinating examples of this phenomenon.
The human visual system also has the ability to adjust itself to varying lighting conditions. When looking at a white piece of paper under incandescent lighting, it will have a decidedly yellow cast. However we have the ability to automatically account for the yellowish light and see the paper as white.