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### Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 5:59 pm
I am in denial with regards that there can't be 2 answers to this question, and don't understand why. Help?
Biologist: We know the following things about plant X. Specimens with fuzzy seeds always have long stems but never have white flowers. Specimens with curled leaves always have white flowers, and specimens with thorny seedpods always have curled leaves. A specimen of plant X in my garden has a long stem and curled leaves.

From the biologist’s statements, which one of the following can be properly inferred about the specimen of plant X in the biologist’s garden?
(A) It has white flowers and thorny seedpods.
(B) It has white flowers but lacks thorny seedpods.
(C) It has white flowers but lacks fuzzy seeds.
(D) It has fuzzy seeds and thorny seedpods.
(E) It lacks both white flowers and fuzzy seeds.

"Definitely C. Because the specimen has curled leaves, we know that it must have white flowers. Because it has white flowers, we know that it cannot have fuzzy seeds. We can't infer anything further."

S/he is correct - it is C. But I don't understand why it can't be A as well. The specimen must have thorny seedpods - why can that not be inferred? Or is s/he wrong in saying that nothing else can be inferred?

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Posted: Fri Nov 26, 2010 6:08 pm
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### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 6:09 pm
fvigaud wrote:I am in denial with regards that there can't be 2 answers to this question, and don't understand why. Help?
Biologist: We know the following things about plant X. Specimens with fuzzy seeds always have long stems but never have white flowers. Specimens with curled leaves always have white flowers, and specimens with thorny seedpods always have curled leaves. A specimen of plant X in my garden has a long stem and curled leaves.

From the biologist’s statements, which one of the following can be properly inferred about the specimen of plant X in the biologist’s garden?
(A) It has white flowers and thorny seedpods.
(B) It has white flowers but lacks thorny seedpods.
(C) It has white flowers but lacks fuzzy seeds.
(D) It has fuzzy seeds and thorny seedpods.
(E) It lacks both white flowers and fuzzy seeds.

"Definitely C. Because the specimen has curled leaves, we know that it must have white flowers. Because it has white flowers, we know that it cannot have fuzzy seeds. We can't infer anything further."

S/he is correct - it is C. But I don't understand why it can't be A as well. The specimen must have thorny seedpods - why can that not be inferred? Or is s/he wrong in saying that nothing else can be inferred?

I remember this exact question It's very condusive to diagramming.

From the stimulus we have,

FS --> LS and -WF

Contraposative states,

WF or -LS --> -FS

Next we have,

CL --> WF

Contraposative,

-WF --> -CL

Next,

TS --> CL

Contraposative

-CL --> -TS

We are told our plant X has both

LS
and
CL

So what do these trigger? If you remember, "triggers" are the sufficient conditions on the left side of our conditional statements. Scanning our previous work we see that LS triggers nothing! So because we have LS we dont know much. If we didnt have LS we would know lots! But the fact that we have it doesnt tell us anything.

Let's look at CL,

If we have CL we have WF but if we have WF we cannot have FS. Thus, answer C is correct because we have WF but not FS.

We dont know about answer choice A because we have no trigger for TS. The conditional statement is,

TS --> CL

And we do have CL, which is neccessary for TS but it is not sufficient to bring about TS. We simply don't know if we have TS or not!

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 6:14 pm
I just PM'd you but in short here's the example i put in the message i sent.

1. If it rained, then the ground is wet.
2. The ground is wet.
3. Therefore, it rained.

It's a logically invalid conclusion because the ground could be wet for any number of reasons.

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 7:18 pm
thank you everybody - totally get it now.
Can't believe I made that mistake again of assuming that the necessary realizes the sufficient. ayayay. So basic.
Thanks so much - helped lots.
Cheers

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 7:55 pm
Anyone have a burning desire to explain why the response to this is C rather than E?
How could C be the overall inference?

Sociologist: Climate and geology determine where human industry can be established. Drastic shifts in climate always result in migrations, and migrations bring about the intermingling of ideas necessary for rapid advances in civilization.
The sociologist’s statements, if true, most strongly support which one of the
following?
(A) Climate is the primary cause of migration.
(B) All shifts in climate produce a net gain in human progress.
(C) A population remains settles only where the climate is fairly stable.
(D) Populations settle in every place where human industry can be established.
(E) Every migrations accompanies by rapid advances in civilization

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 8:32 pm
fvigaud wrote:Anyone have a burning desire to explain why the response to this is C rather than E?
How could C be the overall inference?

Sociologist: Climate and geology determine where human industry can be established. Drastic shifts in climate always result in migrations, and migrations bring about the intermingling of ideas necessary for rapid advances in civilization.
The sociologist’s statements, if true, most strongly support which one of the
following?
(A) Climate is the primary cause of migration.
(B) All shifts in climate produce a net gain in human progress.
(C) A population remains settles only where the climate is fairly stable.
(D) Populations settle in every place where human industry can be established.
(E) Every migrations accompanies by rapid advances in civilization

Again, another one I remember almost by heart

The stimulus says,

Climate Shifts --> migrations --> Ideas NECESSARY for rapid advances in civilization

Also,

Climate and Geology determine where human industry can be established.

A: We don't know that climate is the PRIMARY cause. Just that it might be A cause.

B: All Climate Shifts? The stimulus doesn't say ALL. It says that it brings about a necessary condition but not a sufficient condition.

C: Makes sense because Climate Changes lead to migrations. So a population can only REMAIN settled if the climate is stable.

D: Really out of scope...

E: Again with the EVERY/ALL condition. We know that the necessary condition is brought about but does that necessarily mean that advances happen? Not at all. Just that they can happen.

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 8:54 pm
oooooooook, see it, although I would love to be snarky and reply that we have no proof that where there is no climate change populations don't move Answer C uses 'only' which is pretty strong, and there is no proof. But I see how it is the best choice by process of elimination (especially after you point out that E uses the all/every notion, which shouldn't be necessarily true. thank you.

CS -> M
M -> II
(hence, CS -> II)
CA -> II

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 9:05 pm
fvigaud wrote:oooooooook, see it, although I would love to be snarky and reply that we have no proof that where there is no climate change populations don't move Answer C uses 'only' which is pretty strong, and there is no proof. But I see how it is the best choice by process of elimination (especially after you point out that E uses the all/every notion, which shouldn't be necessarily true. thank you.

CS -> M
M -> II
(hence, CS -> II)
CA -> II

I would say C is pretty well supported. If changes in climate cause populations to migrate (i.e. unsettle) then it is likely that if a population has/remains settled then the climate in that area does not frequently change or else they wouldn't be there.

This question isn't all that condusive to diagramming but what you have is correct,

CS --> Migration --> Necessary Ideas

But the key is not the diagram. It is the term necessary.

### Re: Logic Reasoning - Sufficient & Necessary Question - Help!

Posted: Fri Nov 26, 2010 9:41 pm
ok, thanks very much and have a nice Friday night!