'Now there is one outstandingly important fact regarding Spaceship Earth, and that is that no instruction book came with it.' (Buckminster Fuller)

Here, is a good use of a metaphor. As there is no hard copy guide to life on Earth, this is probably why people create a God in their mind, to whom they turn to for guidence and security. Maybe God is the pilot of our "spaceship" and there is no need for us to have a copy of this "instruction book". But personally, I think that this quote can explain how we get knowledge of things.

How do we know about "Spaceship Earth" to operate it?
When we were first born into our world called Earth, we are clueless about it. However, we come to learn about it by exploring and experiencing, which we would then share this knowledge to others. This means that some of our knowledge came from personal experiences and some from other people informing us.

How do we live on "Spaceship Earth"?
We weren't born with the knowledge of death. So we wouldn't be thinking to survive. But we are born with the instincts on how to survive. We feel hungry and our reasoning would tell us to eat. When a child is drowning, his natural reaction is to kick and paddle. Therefore I believe that we also obtain knowledge from reasoning of the mind.