Logical Reasoning

PT109 · S4 · Q5 Astronaut: Any moon, by definition

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Astronaut: Any moon, by definition, orbits some planet in a solar system.

Conclusion

The moons in solar system S4 all orbit the planet Alpha.

Evidence

Any moon, by definition, orbits some planet in a solar system.

Evaluate

We know that moons always orbit some planet in a solar system, but how would that generic statement prove that all the moons in S4 orbit Alpha?

What would we learn about S4 or Alpha that would guarantee that all the moons in that system orbit Alpha?

Goal

Look for an answer choice that, when combined with the idea that all moons orbit some planet in a solar system, guarantees that all moons in S4 orbit Alpha.

Since there is a new term in the conclusion "S4" and "Alpha", we know the correct answer will have to involve those ideas.

5.

The astronaut's conclusion follows logically if which one of the following is assumed?

  1. There is only one moon

    Nothing About 'Alpha'

    The fact that there's only one moon in S4 doesn't prove that it orbits Alpha. Maybe it orbits Beta, or any other planet in S4.

    0% picked this

  2. Every moon in S4 orbits

    Nothing About 'Alpha'

    The fact that all the moons orbit the same planet doesn't prove that they all orbit Alpha. Maybe they all orbit Beta, or any other planet in S4.

    15% picked this

  3. Correct

    Alpha is the only planet

    Why this is right

    If Alpha is the only planet in solar system S4, and all moons have to orbit some planet in a solar system, then all the moons in solar system S4 must orbit Alpha (it's their only option!)

    Skill tested: Sufficient Assumption · how this choice captures the argument's function is the move to repeat next time.

    78% picked this

  4. Every planet in S4 is

    Nothing About 'Alpha'

    The fact that every planet in S4 has multiple moons doesn't give us any way to prove that all the moons in S4 orbit the same planet, and we'd have no idea whether that planet is Alpha.

    1% picked this

  5. There is at least one

    Too Weak

    This doesn't prove that all the moons in S4 orbit Alpha; it only establishes that at least one moon in S4 orbits Alpha.

    6% picked this

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