"The External World" is Inside the Brain
As a result of the physical facts described so far, we may conclude the following. Everything we perceive as the "matter", "world" or "universe" is nothing but electrical signals occurring in our brain.
Someone eating a fruit in fact confronts not the actual fruit but its perception in the brain. The object considered to be a "fruit" by the person actually consists of an electrical impression in the brain concerning the shape, taste, smell, and texture of the fruit. If the sight nerve travelling to the brain were to be severed suddenly, the image of the fruit would suddenly disappear. Or a disconnection in the nerve traveling from the sensors in the nose to the brain would completely interrupt the sense of smell. Simply put, the fruit is nothing but the interpretation of electrical signals by the brain.
Bundles of light coming from an object fall on the retina upside-down. Here, the image is converted into electrical signals and transmitted to the centre of vision at the back of the brain. Since the brain is insulated from light, it is impossible for light to reach the centre of vision. This means that we view a vast world of light and depth in a tiny spot that is insulated from light.
Another point to be considered is the sense of distance. Distance, which is to say the distance between you and this book, is only a feeling of emptiness formed in your brain. Objects that seem to be distant in that person’s view also exist in the brain. For instance, someone who watches the stars in the sky assumes that they are millions of light-years away from him. Yet what he "sees" are really the stars inside himself, in his centre of vision. While you read these lines, you are, in truth, not inside the room you assume you are in; on the contrary, the room is inside you. Your seeing your body makes you think that you are inside it. However, you must remember that your body, too, is an image formed inside your brain.
The "external world" presented to us by our perceptions is merely a collection of the electrical signals reaching our brain. Throughout our lives, these signals are processed by our brain and we live without recognising that we are mistaken in assuming that these are the original versions of matter existing in the "external world". We are misled because we can never reach the matter itself by means of our senses.
Moreover it is again our brain that interprets and attributes meaning to the signals that we assume to be the "external world". For example, let us consider the sense of hearing. It is in fact our brain that transforms the sound waves in the "external world" into a symphony. That is to say, music is also a perception created by our brain. In the same manner, when we see colours, what reaches our eyes are merely electrical signals of different wavelengths. It is again our brain that transforms these signals into colours. There are no colours in the "external world". Neither is the apple red nor is the sky blue nor the trees green. They are as they are just because we perceive them to be so. The "external world" depends entirely on the perceiver.
Even a slightest defect in the retina of the eye causes colour blindness. Some people perceive blue as green, some red as blue, and some all colours as different tones of grey. At this point, it does not matter whether the object outside is coloured or not. In conclusion, the reason we see objects coloured is not because they are coloured or because they have an independent material existence outside ourselves. The truth of the matter is rather that all the qualities we ascribe to objects are inside us and not in the "external world".
So what remains of the "external world"?