Logical Reasoning

PT105 · S2 · Q16 Brown dwarfs—dim red stars that

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Brown dwarfs—dim red stars that are too cool to burn hydrogen—are very similar in appearance to red dwarf stars, which are just hot enough to burn hydrogen.

Conclusion (accordingly)

If we find a star with no lithium inside, then it is not one of these coolest brown dwarfs.

(Alternatively) All of these coolest brown dwarfs have some lithium inside them.

Evidence

Stars when they're first formed contain lots of lithium. All stars but the coolest brown dwarfs are hot enough to burn off the lithium by converting it into helium.

Evaluate

This is a tough argument to evaluate. You would only really be able to see a gap or think of an objection if you had outside knowledge that stars go through a life cycle, and some of them end up as brown dwarfs, red dwarfs, red supergiants, supernovae, neutron stars, black holes, etc.

The way the facts are presented here make it seem hard to disagree with the conclusion.

If all stars start off with lithium in them, and the coolest brown dwarfs are not hot enough to burn off lithium by converting it into helium, then it feels like the conclusion is right, that any brown dwarf star would still have lithium in it.

I guess one piece of wiggle room is that the premise doesn't say that "brown dwarfs cannot get rid of lithium", it says that But maybe they can get rid of lithium by some other means.

Goal

Let's look for an answer where if we negated it, it would hurt the argument that .

16.

The argument depends on assuming which one of the following?

  1. Correct

    None of the coolest brown

    Why this is right

    If we negate this, it's saying that "some of the coolest brown dwarfs were at some point hot enough to destroy lithium". Does this hurt the argument? Yes, it gives us a pathway to argue that "just because we see a coolest brown dwarf doesn't guarantee there's no lithium ... some of them might have burned up all their lithium back when they were hot enough to destroy lithium." This doesn't feel like the most powerful type of Weaken idea, when negated, because it just creates a possibility of contradicting the conclusion. But that still counts as Weakening. When an author's conclusion is sure of itself, then presenting the possibility that the conclusion could be false is a gamechanger.

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

    62% picked this

  2. Most stars that are too

    Too Specific: most

    "Most" is wrong 99% of the time we see it on Necessary Assumption, because negating most is essentially like toggling between 51% and 49%. Does it make any difference to this author whether 51% vs. 49% of stars that are too cool to burn hydrogen are also too cool to destroy lithium? Nope. It only matters to this author that the coolest brown dwarfs are both too cool to burn hydrogen and do not destroy their lithium completely.

    18% picked this

  3. Brown dwarfs that are not

    Irrelevant Relationship

    If we negate this, it says that brown dwarfs that aren't hot enough to destroy lithium (like the coolest ones in our conclusion) are not hot enough to destroy helium either. Does that weaken? Nope. If the coolest brown dwarfs are cool enough that they can't burn hydrogen, lithium, or helium, that's fine. It's actually weirder for the author to think that cool brown dwarfs are not hot enough for hydrogen / lithium but are hot enough for helium.

    9% picked this

  4. Most stars, when first formed,

    Too Specific

    Too Specific: most Too Strong: roughly the same "Most" is wrong 99% of the time we see it on Necessary Assumption, because negating most is essentially like toggling between 51% and 49%. Does it make any difference to this author whether 51% vs. 49% of stars begin with a roughly equivalent percentage of lithium? Nope. It only matters to this author that whatever percentage of lithium the coolest brown dwarfs start out with, it's big enough that it never gets fully burned off (i.e. whatever lithium they start with, the author just needs it to be true that they never fully deplete it).

    9% picked this

  5. No stars are more similar

    Too Strong: nothing is more similar

    Our spidey-sense should tell us that this is just trying to derive an inference from the very first claim in the paragraph. It has nothing to do with whether we can draw the conclusion in the last sentence. If we negated this and said, "Even though red dwarfs are very similar in appearance to brown dwarfs, there is another type of star that is even more similar in appearance", it wouldn't affect the author's argument at all.

    1% picked this

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