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So, I've realized that - for whatever reason - I'm still having a difficult time grasping necessary vs. sufficient. I've looked at different books and stuff and will think I understand (and I think on the very basic level, I do) but then I'll see an answer choice (or answer choiceS) that heavily rely on the concept of necessary and/or sufficient and feel like a deer caught in headlights. Can anyone help me out by explaining it, maybe in a way different than the usual way it was explained in the Bibles and LSAT Trainer?

i'll try lol..don't know how much of a basic understanding you have, so if my explanation is too basic, I apologize.

necessary - something is required, but not enough by itself, for the conclusion to be true
sufficient - something is enough, but not required, for the conclusion to be true

So, for instance:
for your television to work, it is required that the power cord be plugged in - necessary
For your television to work, no one thing is sufficient to make it work - if it is plugged in, there may be other problems (wires cut, etc.)


So if in an argument, it states that x cannot be done without y, then you know that y is required.
however, you cannot say that x will be done IF you have y, because you may need z and w as well (the argument just didn't mention it).

If an argument, however, says that the presence of A leads to B, then we know that whenever we have A, B follows. However, B could follow from C and/or D as well (the argument just didn't mention it).
For instance, with murder, follows jail time - you know from this that murder is sufficient enough to lead to jail time (although, if you're being pedantic, you could say, its actually proving murder in court, but anyways...), however, other things can just as easily lead to jail time, thus showing that murder doesn't need to be present - stealing, fraud, etc., can just as easily lead to jail time as well.

If this is still too basic, and you already understand the gist of this, you could pm me with what question or specifically what specifically in necessary vs sufficient you don't understand, because your question is really broad.

Just to add on to this. Something can be necessary and sufficient. For example: If and only if the car is red, it will be fast. This means that red cars must be fast (sufficient). It also means only red cars can be fast(Necessary). Interestingly the diagram for if and only if goes both ways. So, if and only if the car is fast, it will be red. This means that if a car is fast it must be red (sufficient). It also means only fast cars can be red (necessary).

You diagram like this: red <--> fast

I know this might confuse you a bit. But basically know that only if you see "If and only if" will it be both.
Otherwise it will follow If (sufficient)->(necessary).

Think of a sufficient condition as an enabler. To dumb it down,
"if sufficient, then it enables or "causes" necessary to be available."

IF (sufficient indicator) a laboratory is destroyed, then it MUST (necessary) be due to a conflagration.
In other words, when a lab is wrecked, you know it had to be because of some fire caused by an accident or something.
However, the reverse is not necessarily true. If a fire occurs, the lab may still stay intact; we don't really know.

The test makers can also switch the order of the conditions.

Susan is REQUIRED to perform a check-up today SINCE she is a doctor.
In this case, the sufficient condition is placed after the necessary.
Because Sue is a doc, you know that a check-up will happen, is going on, or already took place.
The reverse? Sue is doing a check-up? Maybe she's not a doctor, but a medical student.

And then there's "unless." Oh, how I love "unless."

UNLESS there is an open bar, I will not attend the wedding.
This just translates to "I WILL attend the wedding ONLY IF there is an open bar."
Notice how the "will not" transforms into its negated form, and "unless" makes whatever comes after it a necessary condition.
Thus, the open bar bit is the necessary, whereas the attending wedding part is sufficient.
If you're going to the wedding, there's definitely an open bar (yay), but if you're going to the wedding, there might or might not be one (nay).

If Sufficient --> Then Necessary (required, must be)

If you don't understand assumptions, then you don't do well on the LSAT.

Contrapositive (flip and negate)

If you do well on the LSAT, then you understand assumptions.

Thanks for all the responses! I think I've gotten it figured out.... for now.

Deardevil wrote:Think of a sufficient condition as an enabler. To dumb it down,
"if sufficient, then it enables or "causes" necessary to be available."

IF (sufficient indicator) a laboratory is destroyed, then it MUST (necessary) be due to a conflagration.
In other words, when a lab is wrecked, you know it had to be because of some fire caused by an accident or something.
However, the reverse is not necessarily true. If a fire occurs, the lab may still stay intact; we don't really know.

The test makers can also switch the order of the conditions.

Susan is REQUIRED to perform a check-up today SINCE she is a doctor.
In this case, the sufficient condition is placed after the necessary.
Because Sue is a doc, you know that a check-up will happen, is going on, or already took place.
The reverse? Sue is doing a check-up? Maybe she's not a doctor, but a medical student.

And then there's "unless." Oh, how I love "unless."

UNLESS there is an open bar, I will not attend the wedding.
This just translates to "I WILL attend the wedding ONLY IF there is an open bar."
Notice how the "will not" transforms into its negated form, and "unless" makes whatever comes after it a necessary condition.
Thus, the open bar bit is the necessary, whereas the attending wedding part is sufficient.
If you're going to the wedding, there's definitely an open bar (yay), but if you're going to the wedding, there might or might not be one (nay).

Also to add to this "unless" is a necessary indicator in the same way that "no" is a sufficient indicator
Ex: No employee with a morning shift at the ice cream parlor will end up closing the shop.
Can be rewritten as: IF you are on MS (morning shift) you MUST NOT CS (close shop) - Diagrammed as: MS --> notCS
Remember to negate the necessary condition when you see a "no", "no one", "nobody" just as you negate the sufficient condition with "unless"

Its weird for me, I don't know about everyone else, but I have the sufficient- necessary indicators down and very strong conceptual knowledge of how they work but i run into trouble pulling information from the LR text to diagram it. Like if A-->B , B-->C, and C-->D, then we can conclude A-->D shoudlnt be too difficult. Its when they present the same information as notC --> notB, C-->D, notA-->notB, we can still conclude A-->D but this is hard definitely difficult for me to break down efficently enough to not be wasting 3-5 minutes on the question.

Any strategies you guys use to have success on these LR questions?

The one thing I would advise against is getting in the mindset that everything should be diagrammed or is a conditional argument. I was royally screwing myself on Testmasters homeworks trying to find some form of argument when some and many are the much more simple cause and effect type argument.

CoGar wrote:Its weird for me, I don't know about everyone else, but I have the sufficient- necessary indicators down and very strong conceptual knowledge of how they work but i run into trouble pulling information from the LR text to diagram it. Like if A-->B , B-->C, and C-->D, then we can conclude A-->D shoudlnt be too difficult. Its when they present the same information as notC --> notB, C-->D, notA-->notB, we can still conclude A-->D but this is hard definitely difficult for me to break down efficently enough to not be wasting 3-5 minutes on the question.

Any strategies you guys use to have success on these LR questions?

What 34iplaw said; not everything needs to be diagrammed.

Also, can you provide an example? I think we can clear things up better that way.

Deardevil wrote:

CoGar wrote:Its weird for me, I don't know about everyone else, but I have the sufficient- necessary indicators down and very strong conceptual knowledge of how they work but i run into trouble pulling information from the LR text to diagram it. Like if A-->B , B-->C, and C-->D, then we can conclude A-->D shoudlnt be too difficult. Its when they present the same information as notC --> notB, C-->D, notA-->notB, we can still conclude A-->D but this is hard definitely difficult for me to break down efficently enough to not be wasting 3-5 minutes on the question.

Any strategies you guys use to have success on these LR questions?

What 34iplaw said; not everything needs to be diagrammed.

Also, can you provide an example? I think we can clear things up better that way.

I'm at work now, but will try to find the question like this that was driving me up a wall yesterday and get back to ya'll

I had a hard time w/ this too but 7sage cleared it all up for me.

CoGar wrote:

Deardevil wrote:Think of a sufficient condition as an enabler. To dumb it down,
"if sufficient, then it enables or "causes" necessary to be available."

IF (sufficient indicator) a laboratory is destroyed, then it MUST (necessary) be due to a conflagration.
In other words, when a lab is wrecked, you know it had to be because of some fire caused by an accident or something.
However, the reverse is not necessarily true. If a fire occurs, the lab may still stay intact; we don't really know.

The test makers can also switch the order of the conditions.

Susan is REQUIRED to perform a check-up today SINCE she is a doctor.
In this case, the sufficient condition is placed after the necessary.
Because Sue is a doc, you know that a check-up will happen, is going on, or already took place.
The reverse? Sue is doing a check-up? Maybe she's not a doctor, but a medical student.

And then there's "unless." Oh, how I love "unless."

UNLESS there is an open bar, I will not attend the wedding.
This just translates to "I WILL attend the wedding ONLY IF there is an open bar."
Notice how the "will not" transforms into its negated form, and "unless" makes whatever comes after it a necessary condition.
Thus, the open bar bit is the necessary, whereas the attending wedding part is sufficient.
If you're going to the wedding, there's definitely an open bar (yay), but if you're going to the wedding, there might or might not be one (nay).

Also to add to this "unless" is a necessary indicator in the same way that "no" is a sufficient indicator
Ex: No employee with a morning shift at the ice cream parlor will end up closing the shop.
Can be rewritten as: IF you are on MS (morning shift) you MUST NOT CS (close shop) - Diagrammed as: MS --> notCS
Remember to negate the necessary condition when you see a "no", "no one", "nobody" just as you negate the sufficient condition with "unless"

I'm not 100% down with your explanation of unless. In the case of an affirmative followed by an unless, the unless becomes if not, and we reverse the order to put the sufficient condition on the left.

I play pokemon go, unless the servers are down.
-->
I play pokemon go, if not the servers are down.
-->
If not the servers are down, then I play pokemon go.

We can take the contrapositive of our rewritten statement to see that the logical relationship remains intact.

Contrapositive
If not I play pokemon go, then not not the servers are down.

Which put more simply is,

If not I play pokemon go, then the servers are down.

You can see in this case simply making "unless" a necessary condition destroys the logical relationsip of the sentence.

I play pokemon go, unless the servers are down.
!=
If I play pokemon go, then the servers are down.

However in some cases unless does indicate a necessary condition. If we have a "not + unless" combo, not would be dropped, and unless would become then.

I do NOT play pokemon, unless the severs are up.

Becomes,

If I play pokemon, then the servers are up.

Anywho, I hope that helps clarifies unless. There is more to unless, but that'll have to wait until I'm not on mobile.

Probably just repeating people at this point, but...

Necessary- required for something, but not ENOUGH. "Not eating only butter is NECESSARY to live to 100"

Sufficient- not required for something, but ENOUGH for it. "Owning a sick car is sufficient for getting in my pants"

Necessary and Sufficient- both required and enough for something. "I've become so numb to drugs, heroin is both necessary and sufficient to get me high."

Get it?

SweetTort wrote:Probably just repeating people at this point, but...

Necessary- required for something, but not ENOUGH. "Not eating only butter is NECESSARY to live to 100"

Sufficient- not required for something, but ENOUGH for it. "Owning a sick car is sufficient for getting in my pants"

Necessary and Sufficient- both required and enough for something. "I've become so numb to drugs, heroin is both necessary and sufficient to get me high."

Get it?

Very creative last 2 example

Use a car for an example:

For a car to run, it is necessary that it has gas (fuel). In other words, if a car engine is running, I can GUARANTEE you that it has gas

HOWEVER, gas is not sufficient for a car to run. A car can have a full tank of gas, but still not be able to run (aka there can be other issues with the engine).

Gas is necessary, but not sufficient, to the car running.

So, if I said "I drove the car." A necessary assumption is that the car had fuel. Use the denial test, and deny that assumption - if we deny that the car had fuel, the argument falls apart (aka, you could not drive the car if it didn't have any fuel). If you deny a necessary assumption, the argument will ALWAYS fall apart.

http://www.top-law-schools.com/conditio ... oning.html

The link above is a great resource to work some of the kinks out and reinforce the fundamentals.