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#9 - Ornithologist: The curvature of the claws of modern

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Complete Question Explanation

Assumption. The correct answer choice is (B)

The ornithologist states that the curvature of Archaeopteryx’s claws would enable the creature to perch in trees, and concludes that the creature was a tree-dwelling animal.

The paleontologist points out that chickens can perch in trees, but are not tree dwelling, and concludes that the ability to perch in trees is not good evidence that Archaeopteryx was a tree-dwelling animal.

The ornithologist’s argument is essentially a Mistaken Reversal of a commonsense assumption. It makes good sense to suppose that the ability to perch in trees is probably necessary for an animal to be tree-dwelling. However, that does not mean that the ability to perch in trees is sufficient for an animal to be tree-dwelling.

In any case, you are asked to identify a required assumption, so you should focus on the fact that simply because the Archaeopteryx might have perched in trees, the ornithologist concludes that it definitely did so.

Answer choice (A): The ornithologist’s argument concerns only whether the Archaeopteryx was tree-dwelling, not whether it is the ancestor of modern tree-dwelling birds. This off-topic response is incorrect.

Answer choice (B): This is the correct answer choice. The ornithologist concludes that the Archaeopteryx was tree-dwelling, which by any rational standard would involve perching in trees and therefore the use of the creature’s claws. The negation test shows that if the Archaeopteryx did not make use of the curvature of its claws, it would not have perched in trees, so the creature would not be tree-dwelling. That means that this assumption is required by the argument.

Answer choice (C): The fact that all tree-dwelling birds have curved claws does not support the conclusion that all birds with curved claws are tree-dwelling. This response is the commonsense assumption that the ornithologist has mistakenly reversed, and you should avoid making the same error. This choice is wrong, because the ornithologist assumes the Mistaken Reversal of this choice rather than this choice itself.

Answer choice (D): Even if the Archaeopteryx was the earliest birdlike creature, that would not help establish that the Archaeopteryx perched in trees. And from the fact that birds perch in trees today, it is easy to see that having birdlike predecessors would not damage the possibility that a creature perches in trees. This choice is wrong, because it offers no support and, in negation, is still irrelevant.

Answer choice (E): The ornithologist is attempting to show that the Archaeopteryx was a tree-dwelling creature, not that there is only one way to show that the creature was tree-dwelling. In any case, the possibility that there is only one way to argue for a conclusion does not support that the conclusion should be argued for. This choice is incorrect.
Jerrymakehabit
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Can someone help me with choice (E)?

I understand the explanations above that "The ornithologist’s argument is essentially a Mistaken Reversal of a commonsense assumption. It makes good sense to suppose that the ability to perch in trees is probably necessary for an animal to be tree-dwelling. However, that does not mean that the ability to perch in trees is sufficient for an animal to be tree-dwelling."

So the Ornithologist assumes that if curvature of claws enable them to perch, then they are tree-dwelling (which is a mistaken reversal of the premise). (E) says curvature of claws is the only available evidence (for me it means it is sufficient condition) for tree-dwelling, which lines up with Ornithologist's assumption (mistaken reversal of the premise). So I think (E) is correct answer too.

Thanks
Jerry
James Finch
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Hi Jerry,

The issue here is that is that what (E) is stating is not a sufficient condition for the conclusion; just because one piece of evidence is the only one that exists for a certain hypothesis, doesn't mean that piece of evidence is enough to prove the hypothesis correct.

As an example, let's consider a murder suspect I'll call Bob. The police interview Bob based soley on evidence from a witness, who says that Bob killed the victim. Does this mean that Bob is guilty? Not necessarily, as the witness could be biased against Bob, or a pathological liar, or phone records could show Bob was in a totally different place.

At the same time, we should remember that a single piece of evidence can be sufficient: if Bob is caught on tape, face clearly showing, murdering the victim, that would likely be enough to convict him.

So ultimately what (E) is saying is irrelevant, and the idea of it being a sufficient condition should be a clue to that. Assumption questions are looking for necessary assumptions, not sufficient ones (although some correct answers will be both.)

Hope this clears things up!
Jerrymakehabit
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James Finch wrote:Hi Jerry,

The issue here is that is that what (E) is stating is not a sufficient condition for the conclusion; just because one piece of evidence is the only one that exists for a certain hypothesis, doesn't mean that piece of evidence is enough to prove the hypothesis correct.

As an example, let's consider a murder suspect I'll call Bob. The police interview Bob based soley on evidence from a witness, who says that Bob killed the victim. Does this mean that Bob is guilty? Not necessarily, as the witness could be biased against Bob, or a pathological liar, or phone records could show Bob was in a totally different place.

At the same time, we should remember that a single piece of evidence can be sufficient: if Bob is caught on tape, face clearly showing, murdering the victim, that would likely be enough to convict him.

So ultimately what (E) is saying is irrelevant, and the idea of it being a sufficient condition should be a clue to that. Assumption questions are looking for necessary assumptions, not sufficient ones (although some correct answers will be both.)

Hope this clears things up!


Hi James,

Can I understand your explanations as below:

The conclusion of Ornithologist is that if curvature of claws enable them to perch, then they are tree-dwelling (which is a mistaken reversal of the premise). But in (E) "curvature of claws" is NOT sufficient condition for "tree-dwelling ". So (E) does not line up with the conclusion of Ornithologist. If there is an answer choice (F) saying "curvature of claws is the sufficient condition for tree-dwelling", then this choice (F) can be correct for this assumption question. Am I right?

Thanks
Jerry
Brook Miscoski
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Jerry,

No, your proposed (F) "Curvature of claws is the sufficient condition for tree-dwelling" would not be an assumption made by the ornithologist. The ornithologist assumes that the Archeopteryx used its curved claws to live in trees, which is answer choice (B). Let's use the assumption negation technique to see the difference between B and your proposed answer.

B: A did not make use of the curvature of its claws (proves the Ornithologist wrong).
F: Curvature of claws is not a sufficient condition for tree-dwelling (only says that we're unsure whether Ornithologist is right).

Your proposed answer F would be a great Justify answer choice, because your F would be sufficient to prove the Ornithologist correct; but it is a bad Assumption answer choice, because your F is not necessary to allow the Ornithologist's conclusion to be possible.
Jerrymakehabit
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Brook Miscoski wrote:Jerry,

No, your proposed (F) "Curvature of claws is the sufficient condition for tree-dwelling" would not be an assumption made by the ornithologist. The ornithologist assumes that the Archeopteryx used its curved claws to live in trees, which is answer choice (B). Let's use the assumption negation technique to see the difference between B and your proposed answer.

B: A did not make use of the curvature of its claws (proves the Ornithologist wrong).
F: Curvature of claws is not a sufficient condition for tree-dwelling (only says that we're unsure whether Ornithologist is right).

Your proposed answer F would be a great Justify answer choice, because your F would be sufficient to prove the Ornithologist correct; but it is a bad Assumption answer choice, because your F is not necessary to allow the Ornithologist's conclusion to be possible.


Hi Brook,

I am actually more confused now..

In the ornithologist’s argument,the premise is that tree-dwelling :arrow: curve of claw enables them to perch. Then the ornithologist concludes that curve of claw must have enabled them to perch :arrow: tree-dwelling. So the ornithologist’s conclusion is a mistaken reversal of his premise. That is why the ornithologist’s argument is false.

In this question, I thought we need to find the assumption the ornithologist's false reasoning depends on. I was trying to find an assumption that lines up with the ornithologist’s false conclusion which is curve of claw must have enabled them to perch :arrow: tree-dwelling. That is why I created (F) saying "curvature of claws is the sufficient condition for tree-dwelling" which I think could be the correct answer for this question.

I am confused in your analysis, how are we able to use the assumption negation technique here? After negating the assumption, it should hurt the false conclusion?

For example:
"B: A did not make use of the curvature of its claws (proves the Ornithologist wrong)" The Ornithologist's conclusion is wrong anyway. Why do we need the negated assumption to approve it is wrong? Can you please help with my confusion?

Thanks
Jerry
Robert Carroll
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Jerry,

A premise of the argument claims that the curvature enabled the creature to perch. The conclusion is the last statement: the creature was probably tree-dwelling. The Mistaken Reversal is something like "They had the ability to do it. Therefore, they probably did it."

As Brook points out, your hypothetical answer choice (F) would prove that the creature dwelt in trees. This would certainly establish the conclusion, which merely says such a fact was probable, but it's too strong precisely because it proves what the author only claimed in the conclusion was probable.

Every Assumption question deals with an argument that isn't deductively valid. Thus, it's possible for the conclusion to be false even if the premises are true. That's not to say the conclusion is definitely false, just that it could be, because the premises haven't sealed up every weak point of the argument. Here, the conclusion isn't definitely false. It's just that the ornithologist's premises, even if THEY were true, wouldn't be enough to guarantee the conclusion is true. The Assumption Negation technique reveals the usefulness of the correct answer choice. If an answer choice, when negated, destroys the conclusion, it shows that what the answer originally said was so necessary for the argument that its opposite would be fatal to the argument.

Here, hypothetical answer choice (F) isn't necessary for the argument because its negation is compatible with the ornithologist's conclusion. Even if the curvature of the claws were not enough to guarantee a creature is tree-dwelling, that doesn't mean it's not tree-dwelling. A lack of a guarantee is not the same as a disproof. So the argument would survive the negation of (F), showing (F) is not necessary for the argument.

Answer choice (B) is necessary because its negation - "The creature didn't use the curvature of its claws" - destroys the argument that it DID use the curvature to dwell in trees.

Robert Carroll
Jerrymakehabit
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Robert Carroll wrote:Jerry,

A premise of the argument claims that the curvature enabled the creature to perch. The conclusion is the last statement: the creature was probably tree-dwelling. The Mistaken Reversal is something like "They had the ability to do it. Therefore, they probably did it."

As Brook points out, your hypothetical answer choice (F) would prove that the creature dwelt in trees. This would certainly establish the conclusion, which merely says such a fact was probable, but it's too strong precisely because it proves what the author only claimed in the conclusion was probable.

Every Assumption question deals with an argument that isn't deductively valid. Thus, it's possible for the conclusion to be false even if the premises are true. That's not to say the conclusion is definitely false, just that it could be, because the premises haven't sealed up every weak point of the argument. Here, the conclusion isn't definitely false. It's just that the ornithologist's premises, even if THEY were true, wouldn't be enough to guarantee the conclusion is true. The Assumption Negation technique reveals the usefulness of the correct answer choice. If an answer choice, when negated, destroys the conclusion, it shows that what the answer originally said was so necessary for the argument that its opposite would be fatal to the argument.

Here, hypothetical answer choice (F) isn't necessary for the argument because its negation is compatible with the ornithologist's conclusion. Even if the curvature of the claws were not enough to guarantee a creature is tree-dwelling, that doesn't mean it's not tree-dwelling. A lack of a guarantee is not the same as a disproof. So the argument would survive the negation of (F), showing (F) is not necessary for the argument.

Answer choice (B) is necessary because its negation - "The creature didn't use the curvature of its claws" - destroys the argument that it DID use the curvature to dwell in trees.

Robert Carroll



Thanks Robert. I fell a little more clear now.
In the ornithologist’s argument,the premise is that
tree-dwelling :arrow: curve of claw enables them to perch
Then the ornithologist concludes that
curve of claw must have enabled them to perch :arrow: tree-dwelling
So the ornithologist’s conclusion is a mistaken reversal of his premise. That is why the ornithologist’s argument is false.

But I still have the following question.
For the two choices below, can you please explain how to use the negation technique to test on a false reasoning? Since it is already false argument, how does the correct answer (ornithologist's assumption) hurt the false conclusion after negation? Can you please explain using the two choices below? How does the negation of B and F specifically hurt the necessary/sufficient condition in the ornithologist's conclusion (this is my confusion part)?
B: A made use of the curvature of its claws.
F: Curvature of claws is a sufficient condition for tree-dwelling.

Thanks
Jerry
James Finch
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Hi Jerry,

The issue isn't that the argument is "false" per se, but rather that it is incomplete in that it relies upon a set of assumptions that it doesn't spell out. The paleontologist's rebuttal to the ornithologist's argument helps illuminate one of these assumptions: that just because a creature can go to a certain place, doesn't mean that it lives there. I have a key that enables me to enter and exit my office freely; this doesn't make me an office-dwelling creature, since I don't actually live there. I could, I have the key that enables it, but I don't. Archeopteryx has the "key" to perching in trees (curved claws) but this isn't enough to know that it "dwells" in those trees. With that in mind, what is the ornithologist assuming?

I'll diagram the logic in the stimulus as:

Premise 1: Tree-Dwelling Birds :arrow: Perch in Trees :arrow: Curved Claws

Premise 2: Curved ClawsArcheopteryx

Conclusion: Likely Tree-DwellingArcheopteryx

Firstly, the ornithologist is assuming that Archeopteryx uses the claws to perch in trees, and does so enough to be considered to "dwell" in those trees, effectively reversing the stated conditional relationship. This is what I would prephrase to help cut down the answer choices and only test the contenders.

Diagrammed, the assumptions are:

Assumption 1: Curved ClawsArcheopteryx :arrow: Perch in TreesArcheopteryx

Assumption 2: Perch in TreesArcheopteryx :arrow: Tree-DwellingArcheopteryx

We can then use the Assumption Negation technique to test the potential answer choice against the actual conclusion, and see if the answer choice being false will then force that conclusion to be false as well.

(B): Archeopteryx didn't use its claws to perch in trees :arrow: Not likely Archeopteryx was tree-dwelling.

This pretty much destroys the argument; the negation makes sense in that without using its claws, Archeopteryx would have a very hard time living in trees, so it's most likely that it didn't live in trees. It is also a restatement of the first assumption that I Prephrased above.

Contrast this with your hypothetical answer choice:

Curved claws doesn't necessarily mean tree-dwelling :arrow: Not likely Archeopteryx was tree-dwelling

What this is doing is undercutting the relationship between the curved claws and tree-dwelling, but not actually falsifying the conclusion. It's still entirely possible that Archeopteryx was likely tree-dwelling, only that curved claws aren't evidence for it.

Hope this clears things up!
Jerrymakehabit
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James Finch wrote:Hi Jerry,

The issue isn't that the argument is "false" per se, but rather that it is incomplete in that it relies upon a set of assumptions that it doesn't spell out. The paleontologist's rebuttal to the ornithologist's argument helps illuminate one of these assumptions: that just because a creature can go to a certain place, doesn't mean that it lives there. I have a key that enables me to enter and exit my office freely; this doesn't make me an office-dwelling creature, since I don't actually live there. I could, I have the key that enables it, but I don't. Archeopteryx has the "key" to perching in trees (curved claws) but this isn't enough to know that it "dwells" in those trees. With that in mind, what is the ornithologist assuming?

I'll diagram the logic in the stimulus as:

Premise 1: Tree-Dwelling Birds :arrow: Perch in Trees :arrow: Curved Claws

Premise 2: Curved ClawsArcheopteryx

Conclusion: Likely Tree-DwellingArcheopteryx

Firstly, the ornithologist is assuming that Archeopteryx uses the claws to perch in trees, and does so enough to be considered to "dwell" in those trees, effectively reversing the stated conditional relationship. This is what I would prephrase to help cut down the answer choices and only test the contenders.

Diagrammed, the assumptions are:

Assumption 1: Curved ClawsArcheopteryx :arrow: Perch in TreesArcheopteryx

Assumption 2: Perch in TreesArcheopteryx :arrow: Tree-DwellingArcheopteryx

We can then use the Assumption Negation technique to test the potential answer choice against the actual conclusion, and see if the answer choice being false will then force that conclusion to be false as well.

(B): Archeopteryx didn't use its claws to perch in trees :arrow: Not likely Archeopteryx was tree-dwelling.

This pretty much destroys the argument; the negation makes sense in that without using its claws, Archeopteryx would have a very hard time living in trees, so it's most likely that it didn't live in trees. It is also a restatement of the first assumption that I Prephrased above.

Contrast this with your hypothetical answer choice:

Curved claws doesn't necessarily mean tree-dwelling :arrow: Not likely Archeopteryx was tree-dwelling

What this is doing is undercutting the relationship between the curved claws and tree-dwelling, but not actually falsifying the conclusion. It's still entirely possible that Archeopteryx was likely tree-dwelling, only that curved claws aren't evidence for it.

Hope this clears things up!


Hi James,

Thanks for your fast reply. Your diagramming makes so much sense!
I should not have focused on the "mistaken reversal" thing. I should have focused on the "Premise 2" and "Conclusion".
So the conditional chain is
Curved ClawsArcheopteryx :arrow: Perch in TreesArcheopteryx :arrow: Tree-DwellingArcheopteryx (Ornithologist’s Conclusion)

One last question:
I still can not entirely tell the difference between (B) and (F)
(B) Archeopteryx used its claws to perch in trees.
(F) Curvature of claws is a sufficient condition for tree-dwelling.

(B) and (F) both look like a restatement of the conditional chain above. If (B) is correct, why (F) is not correct? Can you please explain just by using the conditional chain we agreed above?

Thanks
Jerry