You're living as part of a small band of hunter-gatherers 30,000 years ago in what will later become France. How do you explain the fact that a stone, when dropped, falls to the earth? How do you even begin to think about gravity?

People sometimes wrinkle their noses when, in the context of explaining the meaning of the word 'theory' in science, you talk about the 'theory of gravity'. Gravity is a fact, isn't it? But from our 21st century perspective, we take too much of 2000 years of theoretical developments for granted. So try to imagine how you would even begin to think about gravity if you were living in a cave 30,000 years ago.

Aristotle