Read the passage.

If experiments and observations show that the moon gravitates toward the Earth, the sea gravitates toward the moon, and the planets gravitate toward one another, we must conclude that the principle of gravitation applies to everything.

–Adapted from "The Rules of Reasoning in Philosophy,” Sir Isaac Newton

What is the central idea of the passage?

Gravity exists because of the moon, the Earth, and the planets.
Experiments and observations show that gravity applies to everything.
Gravity is a type of experiment or observation that applies to all things.
Experiments and observations can describe the moon, the Earth, and the planets.