Professor Williams wrote some statements on the board:

1. Newton observed an apple falling off a tree and speculated that all things on Earth
are subject to the force of gravity.
2. He repeatedly tested his idea.
3. He found that any two objects in the universe have a gravitational attraction to
each other.
4. His findings cannot be edited or replaced.

Professor Williams is discussing

a theory.
a hypothesis.
a law.
an explanation.