Look at this picture.
Clearly, you see that the checker squares labelled A and B are of different shades, right? But check by isolating the squares from the surrounding and you will see that they are in fact the same shade of grey.
What is going on here? From experience you have learned that shadows change the shade of colours they fall upon, and that your mind will compensate for this. Note that it is not your eyes that is “doing” the compensating here, but your mind. Your eyes received the colours and shades as they are, and it is your mind that is doing the adjusting. Remove the surrounding information and you mind will no longer compensate allowing to see the shades as they are.
In Western understanding the mind is not considered a sense faculty. Traditionally there were only thought to be five senses – sight, sound, smell, taste and touch. And the sense corresponding organs were the eyes, ears, nose, tongue and skin. The mind was seen as something different, therefore the brain was not seen as a sense organ.
In Buddhism, the mental objects are sensed by the mind, thus the brain is counted as the sixth sense organ. This has two important implications. Firstly, mental objects (thoughts) are placed on par with the objects of colour and form, sound, scent, flavour and texture, and makes the contents of thought as something ordinary. Secondly, it removes the mind as the seat of some kind of self by default of the ordinariness of what happens in the mind.
In this way we are able to deal with our thoughts and not be ruled by them. Essentially this is what the Buddha’s teaching is all about – taking control of yourself as a physical being – by showing us that the thoughts are really just the real world things that we can control.
Perhaps the self is an illusion as well, much like this image.