Ethan Allen was convinced that every planet out there has its own intelligent extraterrestrials. And this, as you can imagine, is a radical, inspiring, but very unsettling, idea.

Like many of the ideas that mattered in the American Revolution, extraterrestrials got their start in antiquity. The Greek philosopher Epicurus speculated that the universe must be infinite, eternal and abounding in 'worlds' just like our own.

Intelligent life on a planet comes of age when it first works out the reason for its own existence.

For our planet, our world, to come as one, to unite, there shall be an alien, extraterrestrial, that is against us.

Sir Isaac Newton gave the extraterrestrials their biggest shot in the arm when he embraced the infinite universe as the basis for his hugely influential system of physics. Even so, the aliens of the early modern period remained creatures of philosophy rather than science.

If this is the only planet on which not only life, but intelligent life, has arisen, that would be very unusual.