LR Question 17 Explanation
QUESTION TEXT: Although Pluto has an atmosphere and is much larger…
QUESTION TYPE: Sufficient Assumption
CONCLUSION: Pluto is not a true planet.
REASONING: Pluto formed in orbit around Neptune.
ANALYSIS: You must prove that Pluto is not a planet. All we know about Pluto is that it formed in orbit around Neptune.
So the right answer must use that information. For example, we can assume that anything that formed in orbit around a planet can’t itself be a planet.
Many answers point out that Pluto was once a moon. That isn’t enough. Things can change. Those answers would need to say that something can’t be a planet if it was once a moon.
___________
- Pluto was a moon when it was around Neptune. But it wouldn’t be a moon once it left Neptune’s orbit. So it wouldn’t have been simultaneously a moon and a planet.
- This is a necessary assumption. It proves that Pluto could be not a planet. But it doesn’t prove that Pluto is not a planet.
- This doesn’t matter. It’s like saying that if you never aged from the time you were 15 years old, you would still be a teenager. You did age, and Pluto did leave orbit. Things change.
- This doesn’t help us prove that Pluto isn’t a planet. It’s a fact about how we judge whether something is a planet, but it’s not definitive.
- CORRECT. Pluto didn’t form in orbit exclusively around the sun. Therefore, it’s not a planet.
More Resources for Sufficient Assumption Questions
- Conditional Reasoning Article: Learn about conditional statements.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Intro to Conditional Reasoning: Learn conditional reasoning basics.
- Intro Course lesson: This intro course lesson covers Sufficient Assumption questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers sufficient assumption questions.
Hi,
I’m a bit confused about how answer E would be considered a sufficient assumption and not a necessary one. If we negate the answer choice and say “For a celestial body to not be a true planet, it must not have formed in orbit around the sun exclusively”, does this not destroy the argument (and make the answer choice a necessary assumption)? Or am I negating the answer choice incorrectly?
Thank you so much for this resource!
Brian
The negation of (E) would actually be: “for a celestial body to be a true planet it does not need to have formed in orbit around the sun exclusively.” If we plug the above statement back into the argument, the argument does not fall apart. The argument just isn’t impacted at all. So, (E) isn’t necessary in order for the argument to be true.
(E) is sufficient because if we plug it back into the argument, the conclusion smoothly follows from the premises.
Here’s the conditional breakdown of (E) to make that more clear:
True planet –> Formed in orbit around the sun exclusively
~Formed in orbit around the sun exclusively –> ~True planet
Take a look back at the stimulus after reading the above breakdown. Note that the stimulus says that Pluto was not formed in orbit around the sun exclusively, but that it was formed in orbit around Neptune. So, by way of the breakdown we can conclude that Pluto is not a true planet (this is, conveniently, the conclusion of the stimulus.)