There must be an external world, because if there were not something outside me that could glow or reflect light and shine light into my eyes, giving me visual experience, I would not be able to see buildings, crowds, and stars  Moreover, not only do I have such visual experience, but others also have such visual experience
the knowledge of books also repeatedly tells us that there is an external world outside us
Since visual experience is reliable, a mirage is not a so-called illusion, but a real existence.