What key idea does the text below suggest?
Yet so vain is man, and so blinded by his vanity, that no writer, up to the very end of the nineteenth century, expressed any idea that intelligent life might have developed there far, or indeed at all, beyond its earthly level.
Earth's idea of intelligence is insufficient.
Humans are vain and pompous creatures.
Humans are blind to their own shortcomings and weaknesses.
Pride kept humans from imagining life beyond Earth.