## quick formal logic question

Prepare for the LSAT or discuss it with others in this forum.
JJDancer

Posts: 1564
Joined: Sun Jul 26, 2009 7:41 pm

### quick formal logic question

Every B --> A
Every B --> D

Therefore some As are Ds

Is this valid?

NewtonLied

Posts: 77
Joined: Sat Feb 20, 2010 6:16 pm

### Re: quick formal logic question

Yes.

macaulian

Posts: 85
Joined: Wed Jan 20, 2010 6:11 pm

### Re: quick formal logic question

No it is invalid. If I tell you if something is a snozzberry it tastes like steak, and then I tell you that is something is a snozzberry it tastes like applesauce, that does not imply something exists that tastes like steak and tastes like applesauce. That would require that a snozzberry exists.

b->a
b->d
does not imply for some X, a and d, since I did not give you b. This is predicate logic.

NewtonLied

Posts: 77
Joined: Sat Feb 20, 2010 6:16 pm

### Re: quick formal logic question

wrong. snozzberries taste like steak AND applesauce. Therefore some things that taste like steak (snozzberries) also taste like applesauace.

That is as long as you assume snozzberries exist. It depends on if one of the premises of the question is that there is at least one B, and the way it's phrased (every B) makes it sound as though that IS one of the assumptions.
Last edited by NewtonLied on Thu Feb 25, 2010 9:31 pm, edited 1 time in total.

chocho

Posts: 86
Joined: Tue Feb 23, 2010 11:39 pm

### Re: quick formal logic question

i think it would only work if there are B's. without the B to connect them, u cant say there are some with the two characteristics.

basically im trying to say what is above me, but he wrotes its betta.

JJDancer

Posts: 1564
Joined: Sun Jul 26, 2009 7:41 pm

### Re: quick formal logic question

ok so assuming that yes Bs do exist, this would be valid right?
I'm thinking Venn diagram proves this too...

All Bs are As
All Bs are Ds

So All Bs fit inside A (and inside D) and some Ds have to overlap..

yoni45

Posts: 72
Joined: Fri Feb 02, 2007 2:12 am

### Re: quick formal logic question

For the purposes of the LSAT, when given All X are Y, you're allowed to assume that and least 1 X exists.

[ might run counter to some things, but take it as-is for the LSAT, and you'll sleep better at night... =) ]

Given that, yes, it's valid.

Shrimps

Posts: 269
Joined: Sun Feb 14, 2010 10:04 pm

### Re: quick formal logic question

All blueberries are blue
All blueberries are berries

Therefore, there are some blue things that are berries, and there are some berries that are blue.

NewtonLied

Posts: 77
Joined: Sat Feb 20, 2010 6:16 pm

### Re: quick formal logic question

Shrimps wrote:All blueberries are blue
All blueberries are berries

Therefore, there are some blue things that are berries, and there are some berries that are blue.

Ugh, your conclusion is correct, but that logic is atrocious.

iamtaw

Posts: 166
Joined: Wed Dec 23, 2009 3:22 pm

### Re: quick formal logic question

JJDancer wrote:Every B --> A
Every B --> D

Therefore some As are Ds

Is this valid?

No.

If the information you had was:
Every B -> A
Every B -> D
There is at least one B.

in this case you can say that some As are Ds.
You can't claim existence from 2 conditionals

thinkbig

Posts: 253
Joined: Thu Apr 02, 2009 4:59 pm

### Re: quick formal logic question

NewtonLied wrote:
Shrimps wrote:All blueberries are blue
All blueberries are berries

Therefore, there are some blue things that are berries, and there are some berries that are blue.

Ugh, your conclusion is correct, but that logic is atrocious.

you need to assert that blueberries exist. what if there is not at least 1 blueberry?

NewtonLied

Posts: 77
Joined: Sat Feb 20, 2010 6:16 pm

### Re: quick formal logic question

thinkbig wrote:
NewtonLied wrote:
Shrimps wrote:All blueberries are blue
All blueberries are berries

Therefore, there are some blue things that are berries, and there are some berries that are blue.

Ugh, your conclusion is correct, but that logic is atrocious.

you need to assert that blueberries exist. what if there is not at least 1 blueberry?

In most formal logic classes you can usually take an empty set to be the special case. If it isn't defined as empty, you can generally assume it isn't. That's been my experience anyway. I think as far as the LSAT goes it's certainly a safe bet.

SOCRATiC

Posts: 114
Joined: Sun Jan 24, 2010 4:22 am

### Re: quick formal logic question

OMFG.... TITCR:

All B are A
All B are D
Therefore some A are D
-----------

"All B are A" means that some A are B. If some A are B, and if all B are D, then it must be true that some A are in fact D.

Also:Don't pay attention to what anyone else says.
Last edited by SOCRATiC on Thu Feb 25, 2010 10:06 pm, edited 1 time in total.

macaulian

Posts: 85
Joined: Wed Jan 20, 2010 6:11 pm

### Re: quick formal logic question

yoni45 wrote:For the purposes of the LSAT, when given All X are Y, you're allowed to assume that and least 1 X exists.

[ might run counter to some things, but take it as-is for the LSAT, and you'll sleep better at night... =) ]

Given that, yes, it's valid.

I disagree, they can have a LG where they have two conditionals for which the antecedent cannot ever happen. For instance, If ray goes first, then susan goes second, and if ray goes first, jill goes third. If they asked the question that required you to determine if it possible for susan to go second and jill to go third, this could be false. For instance, if I told you susan and jill cannot be placed consecutively (if I give you this, you can conclude that ray never goes first).

Whenever you simplify the real rules of logic, you are making a mistake. There is no such thing as "for LSAT purposes". If they ever asked that question, did not give you that b exists, and the answer was that some a's are d's, people would flip out and they would have to cancel the question, because that is false. Making assumptions that are not given is why alot of people do poorly on the test.

thinkbig

Posts: 253
Joined: Thu Apr 02, 2009 4:59 pm

### Re: quick formal logic question

NewtonLied wrote:
thinkbig wrote:
NewtonLied wrote:
Shrimps wrote:All blueberries are blue
All blueberries are berries

Therefore, there are some blue things that are berries, and there are some berries that are blue.

Ugh, your conclusion is correct, but that logic is atrocious.

you need to assert that blueberries exist. what if there is not at least 1 blueberry?

In most formal logic classes you can usually take an empty set to be the special case. If it isn't defined as empty, you can generally assume it isn't. That's been my experience anyway. I think as far as the LSAT goes it's certainly a safe bet.

The important thing to realize is that, assuming at least 1 blueberry exists, it is true that there are some blue things that are berries. There could be blue things that are not berries, and there could be berries that are not blue. But the conjunction of the two sets guaranties that if all A are B and all A are C, then some B are C.

thinkbig

Posts: 253
Joined: Thu Apr 02, 2009 4:59 pm

### Re: quick formal logic question

SOCRATiC wrote:OMFG.... TITCR:

All B are A
All B are D
Therefore some A are D
-----------

"All B are A" means that some A are B. If some A are B, and if all B are D, then it must be true that some A are in fact D.

Also:Don't pay attention to what anyone else says.

+1. Good explanation.

SOCRATiC

Posts: 114
Joined: Sun Jan 24, 2010 4:22 am

### Re: quick formal logic question

SOCRATiC wrote:OMFG.... TITCR:

All B are A
All B are D
Therefore some A are D
-----------

"All B are A" means that some A are B. If some A are B, and if all B are D, then it must be true that some A are in fact D.

Also:Don't pay attention to what anyone else says.

All Boys are human. --> Some humans are boys.
All Boys are dumb.

Some humans are boys + All boys are dumb = Some humans are dumb.

macaulian

Posts: 85
Joined: Wed Jan 20, 2010 6:11 pm

### Re: quick formal logic question

NewtonLied wrote:
thinkbig wrote:
NewtonLied wrote:
Shrimps wrote:All blueberries are blue
All blueberries are berries

Therefore, there are some blue things that are berries, and there are some berries that are blue.

Ugh, your conclusion is correct, but that logic is atrocious.

you need to assert that blueberries exist. what if there is not at least 1 blueberry?

In most formal logic classes you can usually take an empty set to be the special case. If it isn't defined as empty, you can generally assume it isn't. That's been my experience anyway. I think as far as the LSAT goes it's certainly a safe bet.

Ok, this is very wrong. First of all, this is predicate logic, not (edit) basic symbolic logic, which is often called propositional logic (thanks for the correction). Second, as someone who has taken logic, written papers on the subject, and taught a logic class, I can tell you that anyone who says "empty set" in reference to PL is BSing you. Once again, you cannot assume something exists in logic unless it is given. For example, here is modus ponens:

a->b
a
therefore, b

Without providing a, you cannot prove b. Never assume something not given. Most of the time something may or may not be true, and you are saying that is something may or may not be true, assume that it is true, that is absolutely wrong.
Last edited by macaulian on Thu Feb 25, 2010 10:23 pm, edited 1 time in total.

macaulian

Posts: 85
Joined: Wed Jan 20, 2010 6:11 pm

### Re: quick formal logic question

thinkbig wrote:
SOCRATiC wrote:OMFG.... TITCR:

All B are A
All B are D
Therefore some A are D
-----------

"All B are A" means that some A are B. If some A are B, and if all B are D, then it must be true that some A are in fact D.

Also:Don't pay attention to what anyone else says.

+1. Good explanation.

To bad it is absolutely and utterly wrong. See above. If b->a does not mean there exists a that is b. It never has and never will.

NewtonLied

Posts: 77
Joined: Sat Feb 20, 2010 6:16 pm

### Re: quick formal logic question

macaulian wrote:
yoni45 wrote:For the purposes of the LSAT, when given All X are Y, you're allowed to assume that and least 1 X exists.

[ might run counter to some things, but take it as-is for the LSAT, and you'll sleep better at night... =) ]

Given that, yes, it's valid.

I disagree, they can have a LG where they have two conditionals for which the antecedent cannot ever happen. For instance, If ray goes first, then susan goes second, and if ray goes first, jill goes third. If they asked the question that required you to determine if it possible for susan to go second and jill to go third, this could be false. For instance, if I told you susan and jill cannot be placed consecutively (if I give you this, you can conclude that ray never goes first).

Whenever you simplify the real rules of logic, you are making a mistake. There is no such thing as "for LSAT purposes". If they ever asked that question, did not give you that b exists, and the answer was that some a's are d's, people would flip out and they would have to cancel the question, because that is false. Making assumptions that are not given is why alot of people do poorly on the test.

If they say:
some lawyers are athletes
some lawyers are politicians

and the answer is some politicians are athletes, would you contest that based on the fact that there might not be lawyers? More importantly, would you really not just assume there are lawyers when you answer the question?

thinkbig

Posts: 253
Joined: Thu Apr 02, 2009 4:59 pm

### Re: quick formal logic question

macaulian wrote:
thinkbig wrote:
SOCRATiC wrote:OMFG.... TITCR:

All B are A
All B are D
Therefore some A are D
-----------

"All B are A" means that some A are B. If some A are B, and if all B are D, then it must be true that some A are in fact D.

Also:Don't pay attention to what anyone else says.

+1. Good explanation.

To bad it is absolutely and utterly wrong. See above. If b->a does not mean there exists a that is b. It never has and never will.

I was assuming that B exists. Not safe to assume on the LSAT, but IF any B exists, then it is also A. Therefore, IF any B exists, then there is some A that is also B.

SOCRATiC

Posts: 114
Joined: Sun Jan 24, 2010 4:22 am

### Re: quick formal logic question

macaulian wrote:
Ok, this is very wrong. First of all, this is predicate logic, not symbolic logic. Second, as someone who has taken logic, written papers on the subject, and taught a logic class, I can tell you that anyone who says "empty set" in reference to PL is BSing you. Once again, you cannot assume something exists in logic unless it is given. For example, here is modus ponens:

a->b
a
therefore, b

Without providing a, you cannot prove b. Never assume something not given. Most of the time something may or may not be true, and you are saying that is something may or may not be true, assume that it is true, that is absolutely wrong.

In Symbolic Logic, you have Propositional Calculus AND Predicate Logic. Predicate logic is a subdivision of Symbolic Logic, you dimwit. The demonstration of your ignorance of this fact completely undermines the credibility boost that you attempted to achieve by mentioning your credentials (taking a logic course, writing papers, and teaching the god damned subject).

macaulian

Posts: 85
Joined: Wed Jan 20, 2010 6:11 pm

### Re: quick formal logic question

NewtonLied wrote:
macaulian wrote:
yoni45 wrote:For the purposes of the LSAT, when given All X are Y, you're allowed to assume that and least 1 X exists.

[ might run counter to some things, but take it as-is for the LSAT, and you'll sleep better at night... =) ]

Given that, yes, it's valid.

I disagree, they can have a LG where they have two conditionals for which the antecedent cannot ever happen. For instance, If ray goes first, then susan goes second, and if ray goes first, jill goes third. If they asked the question that required you to determine if it possible for susan to go second and jill to go third, this could be false. For instance, if I told you susan and jill cannot be placed consecutively (if I give you this, you can conclude that ray never goes first).

Whenever you simplify the real rules of logic, you are making a mistake. There is no such thing as "for LSAT purposes". If they ever asked that question, did not give you that b exists, and the answer was that some a's are d's, people would flip out and they would have to cancel the question, because that is false. Making assumptions that are not given is why alot of people do poorly on the test.

If they say:
some lawyers are athletes
some lawyers are politicians

and the answer is some politicians are athletes, would you contest that based on the fact that there might not be lawyers? More importantly, would you really not just assume there are lawyers when you answer the question?

It may be reasonable to assume the existence of a lawyer, but is it reasonable to assume the existance of a snozzberry or a situation where ray goes first? Yes in the first, not in the two examples I have given, so the answer to the question is that for something a and d is true does not follow, as this is general. Just because a line of reasoning works in some specific cases does not mean it is valid. If I can provide one counter example (I have given two), then the reasoning is invalid. Your example proves that you must first determine whether the anticedent of the two conditionals is ever true. By reasoning that lawyers exist your are making that determination.

thinkbig

Posts: 253
Joined: Thu Apr 02, 2009 4:59 pm

### Re: quick formal logic question

NewtonLied wrote:
macaulian wrote:
yoni45 wrote:For the purposes of the LSAT, when given All X are Y, you're allowed to assume that and least 1 X exists.

[ might run counter to some things, but take it as-is for the LSAT, and you'll sleep better at night... =) ]

Given that, yes, it's valid.

I disagree, they can have a LG where they have two conditionals for which the antecedent cannot ever happen. For instance, If ray goes first, then susan goes second, and if ray goes first, jill goes third. If they asked the question that required you to determine if it possible for susan to go second and jill to go third, this could be false. For instance, if I told you susan and jill cannot be placed consecutively (if I give you this, you can conclude that ray never goes first).

Whenever you simplify the real rules of logic, you are making a mistake. There is no such thing as "for LSAT purposes". If they ever asked that question, did not give you that b exists, and the answer was that some a's are d's, people would flip out and they would have to cancel the question, because that is false. Making assumptions that are not given is why alot of people do poorly on the test.

If they say:
some lawyers are athletes
some lawyers are politicians

and the answer is some politicians are athletes, would you contest that based on the fact that there might not be lawyers? More importantly, would you really not just assume there are lawyers when you answer the question?

Good point. Part of the LSAT instructions are that you need to apply standard real world assumptions. You have to assume that lawyers exist. If it is a special specific case (If Jim is in line before Julia, then Jim is third) then you can;t assume that the antecedent is extant unless it is declared.

iamtaw

Posts: 166
Joined: Wed Dec 23, 2009 3:22 pm

### Re: quick formal logic question

NewtonLied wrote:
macaulian wrote:
yoni45 wrote:For the purposes of the LSAT, when given All X are Y, you're allowed to assume that and least 1 X exists.

[ might run counter to some things, but take it as-is for the LSAT, and you'll sleep better at night... =) ]

Given that, yes, it's valid.

I disagree, they can have a LG where they have two conditionals for which the antecedent cannot ever happen. For instance, If ray goes first, then susan goes second, and if ray goes first, jill goes third. If they asked the question that required you to determine if it possible for susan to go second and jill to go third, this could be false. For instance, if I told you susan and jill cannot be placed consecutively (if I give you this, you can conclude that ray never goes first).

Whenever you simplify the real rules of logic, you are making a mistake. There is no such thing as "for LSAT purposes". If they ever asked that question, did not give you that b exists, and the answer was that some a's are d's, people would flip out and they would have to cancel the question, because that is false. Making assumptions that are not given is why alot of people do poorly on the test.

If they say:
some lawyers are athletes
some lawyers are politicians

and the answer is some politicians are athletes, would you contest that based on the fact that there might not be lawyers? More importantly, would you really not just assume there are lawyers when you answer the question?

also just by purely using logical language,
if you just have
1. B -->A
2. B -->D
you cannot get the conclusion
A&D

Return to â€œLSAT Prep and Discussion Forumâ€?

### Who is online

Users browsing this forum: No registered users and 8 guests