#20 - The study of primates is interesting for many reasons, including the fact

[Luke Haqq]

Complete Question Explanation

Must Be True. The correct answer choice is (B).

This stimulus can be broken into several conditional statements. First, we are told that "only primates have opposable thumbs." This can be diagrammed as:

Opposable thumbs Primate

We're also told that "lemurs are lower primates and the only primates indigenous to Madagascar." This can be diagrammed as:

Lemur (lower) primate

Primate indigenous to Madagascar Lemur

We're told that "Some species of lemurs are the only living lower primates that are diurnal," or:

Lemur diurnal

Finally, we're told, "All higher primates are thought to have evolved from a single diurnal species of lower primates."

(Higher) primate evolved from diurnal lower primate

Answer choice (A): We don't know this to be true. We know that higher primates, including the chimpanzee, evolved from diurnal lower primates. We also know that lemurs are lower primates and some of them are diurnal. But we don't know that they are the specific lower primate from which chimpanzees evolved (it might have been a different diurnal lower primate).

Answer choice (B): This is the correct answer choice. This follows from a combination of the second and third piece of conditional reasoning above:

Primate indigenous to Madagascar Lemur (lower) primate

That is, if a primate is indigenous to Madagascar, then we know that primate is a lemur, which is a lower primate.

Answer choice (C): We don't know this, and the stimulus doesn't mention nocturnal at all. Rather, it only mentions that some lemurs "are the only living --lower-- primates that are diurnal." So that suggests the rest of lower primates are nocturnal (since they are not diurnal). However, this answer choice is a claim about --higher- primates being nocturnal, which we don't have information about in the stimulus.

Answer choice (D): We know that if a creature has opposable thumbs, then it is a primate. However, we don't know this because the stimulus does not tell us about those without thumbs. It's possible based on the material that some primates don't have thumbs, but the stimulus doesn't say one way or another.

Answer choice (E): We don't know this. We know that some lemurs are diurnal, suggesting that the others are nocturnal. The stimulus would have to say something like "all" lemurs are diurnal for it to be true that there are no nocturnal lemurs.

[mseggio]

Don't let answer choice A fool you....

Lets look at the following:
(1) the Lemurs are the ONLY species of primates indigenous to the island of Madagascar (awesome, good to know LSAC)
(2) ALL the higher primates have evolved from a single species of diurnal lower primates (once again, thank you LSAC)
(3) But, only SOME species of lemurs are diurnal lower primates (stop "monkeying" around LSAC)

So, not only is it possible that the species of Lemur native to Madagascar ar:
(1) not diurnal, but it could also be inferred that
(2) they're the ONLY indigenous species of primates to the island, so any indigenous HIGHER species could not also be indigenous to the same island.

Answer choice analysis:
A - based on the information I believe this could be inferred, but is it the "best" answer choice (IDK, we'll save it for later)
B - oh look, this discusses that concept that no higher primates are indigenous to the island because the singular Lemur species is the ONLY species of primate (higher or lower) which are native to the island
C - if you picked this, you probably thought about how ALL higher primates evolved from a singular species of lower, diurnal primate (which again, I don't think this is a bad observation, but is it the best?)
D - if only primates have opposable thumbs and ANY Lemur is ALWAYS a primate, then this isn't true
E - this can't be true either because SOME Lemurs are diurnal and OTHERS ARE NO, so those "some" Lemurs which are not diurnal could still be nocturnal

[Robert Carroll]

mseggio,

There is simply no evidence for answer choice (A) or answer choice (C).

For answer choice (A), all higher primates are thought to have evolved from a single diurnal species of lower primates - was that single species a lemur? There is no indication of that whatsoever in the stimulus, so that answer reaches a dead end immediately.

Answer choice (C) is also completely unfounded. All higher primates are thought to have evolved from a diurnal primate. But...why would they still be diurnal? There's no way to know.

For answer choice (D), we actually don't know this is true, but it may be. Nothing in the stimulus says that all primates have opposable thumbs. That's simply a Mistaken Reversal of "only primates have opposable thumbs". So this answer choice reaches an impasse where we can't infer it without maybe using a Mistaken Reversal.

Because "some" can include "all", answer choice (E) isn't definitely false, but, more importantly, it's not definitely true. Therefore, it's wrong for this Must Be True question.

Robert Carroll

[justlikemagic]

I had originally chosen C but I get why it's wrong and why B is correct.

I had a question on D), is it basically " some lemurs are 'missing' opposable thumbs" or can it be translated to:
" if you're a lemur, then some of you do not have opposable thumbs" which in turn would be:
lemur ~opposable thumbs

for a long time I was stumped on this because "without" had messed with me big time. when I see "unless" I totally got it, but "without" - I was having trouble making out how I'd diagram it even though the way to do so is the same between both words. For a while I thought it was: lemur opposable thumbs but I knew it wasn't that because it didn't accurately represent the sentence. rather, that diagram represents "if i'm a lemur, then some of me have opposable thumbs" - any help on how to ease this confusion? is it hard because the sentence isn't a true conditional statement?

i know the way to diagram the unless equation but I was just so confused with this for whatever reason. is it even something one could diagram or is it just a statement that isn't diagrammable?
(i know it's not necessary to diagram it to know it's wrong, I never thought it was correct to begin with, I am just asking about D) as a standalone thing - not in relation to the stimulus (if that makes sense)

and with C) it's:
higher primate ~nocturnal

and would that equal to:
higher primate diurnal

and for A)
I know it's incorrect and unfounded but I wanted some clarification as the reasoning confused me a bit.

"For answer choice (A), all higher primates are thought to have evolved from a single diurnal species of lower primates - was that single species a lemur? There is no indication of that whatsoever in the stimulus, so that answer reaches a dead end immediately."

the stimulus says that "some species of lemurs are the only living lower primates that are diurnal" wouldn't that be -
lower primate that's diurnal species of lemurs

and A) is about a chimp, which is a higher primate, having evolved from the lemur. (aside from not even knowing if that thought is even true to begin with)ohhh, wait. - is that what you were implying with your explanation, Robert??


(but let's say the 'thought' is in fact true, and they DO evolve from a single diurnal species of lower primates) wouldn't the reason that A) is unfounded is because we don't know if they evolved from one of the species of lemurs that are diurnal? or is that not necessarily the case, since the sentence said "the only 'living' lower primates...diurnal". Whereas the conclusion didn't state 'living,' in which case the chimps could have evolved from a lower primate who's diurnal but is maybe extinct and isn't living anymore?

[Adam Tyson]

I had a question on D), is it basically " some lemurs are 'missing' opposable thumbs" or can it be translated to:
" if you're a lemur, then some of you do not have opposable thumbs" which in turn would be:
lemur ~opposable thumbs

I would say these are two ways of saying the same thing, justlikemagic, and they are both correct. Good work! It means that there is at least one lemur that does not have opposable thumbs, so you could say they are missing them.

and with C) it's:
higher primate ~nocturnal

That part is correct! But this next one is not:

and would that equal to:
higher primate diurnal

Because there may be things that are neither diurnal nor nocturnal. What if they are active equally during day and night? Don't assume that "not X" means "the polar opposite of X." There may be other alternatives along a whole spectrum of possibilities!

Looks like you got the rest all correct! There may have been other diurnal lower primates besides lemurs; it's just that lemurs are the only ones that are still around today.

[justlikemagic]

gotcha, thank you. I wasn't sure if diurnal and nocturnal were binary or not. but it makes sense, of course there is the possibility of being equally awake in the day and night. and thank you for the info on A).

for D, could you give me an example of a sentence that has both "some" and "without" in which the necessary actually isn't negated. You know when you say something so many times it doesn't make sense anymore, I kind of felt like that with D) (not totally sure why) in terms of not seeing that the negation would be in front of the necessary condition. I'd say I'm good at conditionals, but I think 'without' had my mind go blank. If possible, I wanted to see the difference between a sentence containing those two words, where the necessary would be negated vs when the necessary is not negated.

[Rachael Wilkenfeld]

Hi justlikemagic,

Let's look at a few different without statements.

No one without a license can drive a car.

Drive licence

Here's the key. The "no one" is negative. The "without" is negative (that's why we use the unless equation). Combining the two negative gets us the positive "drive."

Someone without a licence can't drive a car

Drive licence

It's the same concept here as above. They mean the same thing. If you drive a car, you need a licence.

There are lots of ways to express the same idea conditionally. You can use the unless equation with the without conditionals, but you can also get there in other ways. For example, you can think about these statements as if---then statements, considering which term is the "if" term and which is the "then" term.

Hope that helps!

[sbose]

Hi there!

I guessed and got this question correct, but I'm fairly confident I misidagrammed either the stimulus and/or answer choice B, because one of the diagrams would be a mistaken negation or reversal of the other.

Stimulus

The study of primates is interesting for many reasons, including the fact that only primates have opposable thumbs:

Opposable thumbs Primates

The lemurs are lower primates and the only primates indigenous to Madagascar, a large island off the coast of southeastern Africa.

Lemurs Lower Primates

Indigenous to Madagascar Lemurs (this is the statement I had the most trouble with)

Some species of lemurs are the only living lower primates that are diurnal—that is, active primarily during the day.

Species of lemur living lower primates that are diurnal

All higher primates are thought to have evolved from a single diurnal species of lower primates.

Higher primates evolved from a single diurnal species of lower primates

Answer Choice B

No primates indigenous to Madagascar are diurnal higher primates.

Primate indigenous to Madagascar NOT diurnal higher primates

[Robert Carroll]

sbose,

The diagramming looks good overall, but doesn't it prove answer choice (B) correct?

Indigenous to Madagascar Lemurs

and

Lemurs Lower Primates

Means that the only primates indigenous to Madagascar are lower, not higher, primates.

Robert Carroll