Passage Summary: Stars start with lithium, but most burn it up. Only the very coldest brown dwarfs are too chilly to burn it. So, if you find a star that has already used up all its lithium, the argument says it can't be one of those cold ones.

Conclusion: Any star discovered that lacks lithium cannot be categorized as one of the coolest brown dwarfs.

Reasoning: Stars begin with lithium, and every type of star except the coolest brown dwarfs is hot enough to destroy that lithium by converting it to helium.

Analysis: The argument relies on a 'Gap' regarding how lithium disappears. We are told that heat is a sufficient condition to destroy lithium, and that the coolest brown dwarfs lack this heat. However, for the conclusion to be certain, we must assume that heat is the *only* way a star could lose its lithium. If the coolest brown dwarfs had some other way to get rid of lithium—perhaps through a different chemical process or by ejecting it—then finding one without lithium wouldn't prove it isn't a cool brown dwarf. Look for an assumption that eliminates other ways these specific stars might lose their lithium.