Passage Summary: Newton's rules say gravity only cares about how heavy things are and how far apart they are. Because of this, the author concludes you don't need to know what a planet is made of to figure out its path through space.

Conclusion: Calculating the orbits of the Sun and planets does not require a theory regarding their internal structure or composition.

Reasoning: Newtonian theory states that gravitational force depends only on the mass of bodies and the distance between them, regardless of what they are made of.

Analysis: The argument assumes that gravity is the only factor needed to calculate an orbit. While the premise establishes that gravity is independent of a body's composition, the conclusion assumes that nothing *else* involved in calculating an orbit depends on that composition. To make this logically airtight, we need an assumption that confirms orbits are determined exclusively by the gravitational forces described. If something else—like magnetic fields or internal density distribution—affected the orbit, the conclusion would fail.