When you are deeply engaged in solving a mathematical problem, you do not hear any sound though your friends shout loudly by your side. Tram-cars and motor-cars run about in the street. If you are deeply engaged in something, you do not hear them. Why? Because your mind was not linked with the organ of hearing. The indriyas cannot do anything by themselves. They need the help of their master, the mind, at every moment. It is the mind that sees, hears, tastes, smells and feels. Senses simply transmit the sense-vibrations from outside to the mind.
The table that you see is a mental kalpana (imagination). This table, though you see it externally, really exists in your mind. A mental picture, or image, plus external something is the table. That which exists outside is unknown and unknowable. Mind reacts to external vibrations and you see the external universe. If there is no mind there is no universe. Where is the world for you during deep sleep? Hence this world is termed as manomatra jagat, or manahkalpita jagat. This mind is the universe. There is no world apart from the mind.
How can a tiny lens produce a big image of a mountain? A small image is produced by the lens, the retina and the optic centre. It is the mind that develops and enlarges it. All the pictures do already exist in the mind. What you perceive as a big mountain outside is only the mental image plus some external unknowable something.
If you say: ‘there is nothing outside’ why do you run after food and water when you are hungry and thirsty? If everything is within, you should get satisfaction from hunger and thirst from within only. But it is not the case. There is something outside, call it a vivarta (appearance) of Brahman, vilasa (expansion) of maya, camatkara (trick) of avidya (ignorance), pariṇama (modification) of Vishnu, electrical waves of scientists, combination of anus (atoms) of Kanada, or tatva (elemental) vilasa. It does not matter much.