This actually wasn’t obvious at all. If I let go of an apple in midair, it falls. Why? Nothing appeared to be acting on it. The “common sense” explanation is that things naturally fall. Their “default” action is to move toward the earth. That’s why there are explanations from ancient myths about the sun and stars being “hung” in the sky. Cause otherwise, they would fall to earth too, right? Everything does.

What Newton did was to show that there is a force acting on the apple, and without that force, it wouldn’t move. He also came up with an equation that could predict what that force would be between any two objects at any distance, and what motion or lack of motion would result from that force.