I've just started reading Coming Back to Earth by James Clarke and he quotes astronaut Frank Borman, commander of the Apollo 8, which was the first manned spacecraft to fly around the moon.

When you are privileged to view the Earth from afar, when you can hold out your thumb and cover it with your thumbnail, you realise that we are really, all of us around the world, crew members on the space station Earth. Of all the accomplishments of technology, perhaps the most significant one was the picture of Earth over the lunar horizon. If nothing else, it should impress our fellow man with the absolute fact that our environment is bounded, that our resources are limited, and that our life support system is a closed cycle. And, of course, when this space station is viewed from 240,000 miles away, only its beauty, its minuteness, and its isolation in the blackness of space, are apparent. A traveller from some far planet would not know that the size of the crew is already too large and threatening to expand, that the breathing system is rapidly becoming polluted, and that the water supply is in danger of contamination with everything from DDT to raw sewage. The only real recourse is for each of us to realise that the elements we have are not inexhaustible. We’re all in the same spaceship.