# A is B but B isn't A



## ans7

Why is a cat a mammal but a mammal isn't a cat.

How does "is" work?


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## dojibear

Cats are a subset of mammals. So all cats are mammmals.

Mammals are not a subset of cats. Some mammals are not cats. Some mammals are cats.


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## Uncle Jack

What is it that you do not understand? The first clause is inverted because this is a question, so as statements, the two clauses are:
A cat is a mammal.​A mammal isn't a cat.​The verb "be" is used to say something about the subject. Where the subject is a singular noun representing a whole class of things, then it is taken to be a general statement, something along the line of "most or all cats" or "most or all mammals".


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## ans7

Uncle Jack said:


> What is it that you do not understand? The first clause is inverted because this is a question, so as statements, the two clauses are:
> A cat is a mammal.​A mammal isn't a cat.​The verb "be" is used to say something about the subject. Where the subject is a singular noun representing a whole class of things, then it is taken to be a general statement, something along the line of "most or all cats" or "most or all mammals".


what i don’t understand is how do one say something about the subject? what is “a mammal” to “a cat”?


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## elroy

A mammal is what a cat is.


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## heypresto

ans7 said:


> what i don’t understand is how do one say something about the subject? what is “a mammal” to “a cat”?


A cat is a mammal. But not all mammals are cats. So we can't say 'a mammal is a cat.

This is simple logic, not a grammar or language question.


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## ans7

heypresto said:


> A cat is a mammal. But not all mammals are cats. So we can't say 'a mammal is a cat.
> 
> This is simple logic, not a grammar or language question.


what is this logic? when you just think about it, it makes sense but when you think deep enough it doesn’t.

are a mammal and a cat the same thing? do they have equal matters in them? I dont get it


elroy said:


> A mammal is what a cat is.


thanks that really helps yeah


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## elroy

ans7 said:


> are a mammal and a cat the same thing? do they have equal matter in them i dont get it


I’m positive the relationship between the two is the same in Vietnamese.  If you don’t understand it even when thinking about the Vietnamese words, what you need is a biology or philosophy forum, not a language forum.


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## danieleferrari

Mammals (hyperonym) > Cats (hyponym)


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## suzi br

It is also a matter of taxonomy; the science of classification.  It is part of what you learn in biology but also in all categories of naming anything in language.

You start with a big group - mammals - these are all animals with warm blood and fur and milk-fed babies.

Within that are lots of sub-sets. 
For example cats or dogs or humans
Within cats there are tigers, tabbies, lions etc.

All the tigers are cats, all the cats are mammals.
All the humans are mammals.
All the cats and all the humans are mammals.  

Not all the cats are tigers. 
Not all mammals are cats, they might be dogs or humans or any other animal with warm blood, fur and milk-fed babies.


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## danieleferrari




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## suzi br

danieleferrari said:


> Mammals (hyperonym) > Cats (hyponym)
> 
> Cats < Mammals
> 
> Animals > Mammals > Cats...


Thanks I knew I knew this vocabulary.  

It is a fascinating subject in language, as well as other sciences, as you see when you work with small children and watch them learn which items belong in which name.  Lot of kid's first books focus on this classification skill.  

Or this classic:

Daddy is a man ...  but not all men are daddy.


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## ans7

danieleferrari said:


> Mammals (hyperonym) > Cats (hyponym)





suzi br said:


> It is also a matter of taxonomy; the science of classification.  It is part of what you learn in biology but also in all categories of naming anything in language.
> 
> You start with a big group - mammals - these are all animals with warm blood and fur and milk-fed babies.
> 
> Within that are lots of sub-sets.
> For example cats or dogs or humans
> Within cats there are tigers, tabbies, lions etc.
> 
> All the tigers are cats, all the cats are mammals.
> All the humans are mammals.
> All the cats and all the humans are mammals.
> 
> Not all the cats are tigers.
> Not all mammals are cats, they might be dogs or humans or any other animal with warm blood, fur and milk-fed babies.





danieleferrari said:


> View attachment 76011





suzi br said:


> Thanks I knew I knew this vocabulary.
> 
> It is a fascinating subject in language, as well as other sciences, as you see when you work with small children and watch them learn which items belong in which name.  Lot of kid's first books focus on this classification skill.
> 
> Or this classic:
> 
> Daddy is a man ...  but not all men are daddy.


Well the problem is that there are times when A isn't a subset of B.

My dad is the man
The man is my dad


He is one man
One man is him

And then there's the articles: a/an
A mammal is a cat OR a tiger OR a human OR...........
This basically means a mammal is a cat ight? That's what the conjunction "or" does


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## suzi br

Wrong.

I think we have probably done all we can in English.
I refer you to posts #6 and #8.


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## ans7

suzi br said:


> Wrong.
> 
> I think we have probably done all we can in English.
> I refer you to posts #6 and #8.


It's because of languages that this is a problem, more specifically because of the linking verb "be", the verb itself creates a whole new logic that shouldn't exist


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## suzi br

No-one else can see the problem, so you are going to have to try harder to express what you think the problem is!


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## suzi br

ans7 said:


> It's because of languages that this is a problem, more specifically because of the linking verb "be", the verb itself creates a whole new logic that shouldn't exist


It's probably more to do with the choice of article than the verb.


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## Edinburgher

I know exactly what the problem is.  Or at least I think I do.  As far as I can tell, the question is simply about what exactly "is" means (as Clinton once said).

When we say that A *is* B, this generally doesn't correspond to mathematical or logical equality.
We can say that three plus five *is* eight, as a less formal way of saying that it *equals* eight.

Mathematical and logical equality is symmetric, and so when A=B is true, then B=A is also true.
The same is not the case with normal language "is".  When A is B, it is generally not the case that B is A.
We can describe A as B, but not the other way round. The relation is in general not symmetric.

So:
A cat is a mammal.
A dog is a mammal.

Those sentences do not mean a mammal is a cat, or a mammal is a dog, because then a dog would be a cat.


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## Keith Bradford

In formal logic this (apparent) paradox is avoided by using "all" or "some" instead of "a".

All cats are mammals.
Some mammals are cats.
⇒ No paradox.


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## ans7

Edinburgher said:


> I know exactly what the problem is.  Or at least I think I do.  As far as I can tell, the question is simply about what exactly "is" means (as Clinton once said).
> 
> When we say that A *is* B, this generally doesn't correspond to mathematical or logical equality.
> We can say that three plus five *is* eight, as a less formal way of saying that it *equals* eight.
> 
> Mathematical and logical equality is symmetric, and so when A=B is true, then B=A is also true.
> The same is not the case with normal language "is".  When A is B, it is generally not the case that B is A.
> We can describe A as B, but not the other way round. The relation is in general not symmetric.
> 
> So:
> A cat is a mammal.
> A dog is a mammal.
> 
> Those sentences do not mean a mammal is a cat, or a mammal is a dog, because then a dog would be a cat.


Yes that's half of the problem, "be" does not mean "="
The second half is what "be" does if it isn't "="


Keith Bradford said:


> In formal logic this (apparent) paradox is avoided by using "all" or "some" instead of "a".
> 
> All cats are mammals.
> Some mammals are cats.
> ⇒ No paradox.


But I want to use "a/an", the whole problem is that I can't use a and an


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## PaulQ

ans7 said:


> Why is a cat a mammal but a mammal isn't a cat.


I think your question is based upon your being taught that "to be" (is) is a copular verb and that the subject and complement should be reversible. This is nonsense and should not be taught.


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## suzi br

ans7 said:


> what "be" does if it isn't "="


Look in the dictionary, esp definition #5 !

be - WordReference.com Dictionary of English


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## ans7

PaulQ said:


> I think your question is based upon your being taught that "to be" (is) is a copular verb and that the subject and complement should be reversible. This is nonsense and should not be taught.


Maybe I was taught that maybe I wasn't, but now I know that it's not reversible and now I just want to know how "be" works


suzi br said:


> Look in the dictionary, esp definition #5 !
> 
> be - WordReference.com Dictionary of English


A cat is a mammal

Is "a" mammal a group?


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## heypresto

ans7 said:


> But I want to use "a/an", the whole problem is that I can't use a and an


A cat is a (type of) mammal. 
A mammal is a (type of) cat. 

An orange is a (type of) fruit. 
A fruit is a (type of) orange.


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## heypresto

ans7 said:


> A cat is a mammal
> 
> Is "a" mammal a group?


Here 'a cat' means all cats, and 'a mammal' means all the animals that are mammals.


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## PaulQ

ans7 said:


> Is "a" mammal a group?


No. 
A mammal is a member of a group.

The group is "mammals".


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## ans7

heypresto said:


> Here 'a cat' means all cats, and 'a mammal' means all the animals that are mammals.


“all cats” “all the animals”

so a cat or a mammal is plural singularly?

i dont think it means all cats it means one of all cats


PaulQ said:


> No.
> A mammal is a member of a group.
> 
> The group is "mammals".


I know


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## Edinburgher

ans7 said:


> so a cat or a mammal is plural singularly?


No.  "*A* cat" is *one* cat.  But here it doesn't mean one specific cat, but any arbitrary cat (any member of the set of all cats).
Because it means any cat, whatever we say about it applies to all cats.

So "a cat is a mammal" in effect means that all cats are mammals: all members of the set of all cats are also members of the set of all mammals, or the set of all cats is a subset of the set of all mammals.


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## PaulQ

ans7 said:


> I know


Good. You know that a member of a group possesses defining characteristics, and you know that one member of a group is not necessarily definitive of the group, it may have other, distinguishing features. So what is the problem?


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## zhg

Maybe OP was thinking how can we know what does "is" mean, in "A is B, but B isn't A".

For example when we plug in, cat=A, mammal=B. The statement  "A is B, but B isn't A" is true, and A is B is not reversible.
But when we put something else into that statement, like, "his name"=A, "Alex" = B. The statement "A is B" becomes reversible.


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## ans7

Edinburgher said:


> No.  "*A* cat" is *one* cat.  But here it doesn't mean one specific cat, but any arbitrary cat (any member of the set of all cats).
> Because it means any cat, whatever we say about it applies to all cats.
> 
> So "a cat is a mammal" in effect means that all cats are mammals: all members of the set of all cats are also members of the set of all mammals, or the set of all cats is a subset of the set of all mammals.


From what i’ve gathered in this thread, if A belongs to class B, A is B.

Mammals are a class, a cat belongs to that class, does that mean: A cat is mammals?


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## suzi br

ans7 said:


> From what i’ve gathered in this thread, if A belongs to class B, A is B.
> 
> Mammals are a class, a cat belongs to that class, does that mean: A cat is mammals?


No. You need an article and NOT a plural on the last word.

If you are going to do this right, you need absolute accuracy.

A cat is *a* mammal.


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## heypresto

ans7 said:


> does that mean: A cat is mammals?


No, it means a cat is a _type of_ mammal. It's in the class of animals we call 'mammals.'

A mammal, however, isn't a type of cat. It's not in the class of animals that we call 'cats.'


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## ans7

zhg said:


> Maybe OP was thinking how can we know what does "is" mean, in "A is B, but B isn't A".
> 
> For example when we plug in, cat=A, mammal=B. The statement  "A is B, but B isn't A" is true, and A is B is not reversible.
> But when we put something else into that statement, like, "his name"=A, "Alex" = B. The statement "A is B" becomes reversible.


Yes that's one of "be"'s functions.

It can talk about the state: John is sad
It can talk about the location: John is at my house
It can mean equal: John is my dad/my dad is John
And it can mean whatever we are discussing: John is a man/a man is John (wrong)


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## suzi br

zhg said:


> Maybe OP was thinking how can we know what does "is" mean, in "A is B, but B isn't A".
> 
> For example when we plug in, cat=A, mammal=B. The statement  "A is B, but B isn't A" is true, and A is B is not reversible.
> But when we put something else into that statement, like, "his name"=A, "Alex" = B. The statement "A is B" becomes reversible.


Maybe, see #21 above.


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## ans7

suzi br said:


> No. You need an article and NOT a plural on the last word.
> 
> If you are going to do this right, you need absolute accuracy.
> 
> A cat is *a* mammal.


Yes I know we need the articles but why is that? Don't the rules say if A is a part of B, A is B? A cat is definitely a part of mammals


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## Edinburgher

Yes, but here the symbols 'A' and 'B' stand for noun phrases that would usually include the indefinite article.

Also, saying "is part of mammals" is ungrammatical.  You can say either "is part of *the set of* mammals" or "is a mammal".


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## ans7

Edinburgher said:


> Yes, but here the symbols 'A' and 'B' stand for noun phrases that would usually include the indefinite article.
> 
> Also, saying "is part of mammals" is ungrammatical.  You can say either "is part of *the set of* mammals" or "is a mammal".


Yes it's ungrammatical this whole thread is ungrammatical and we are discussing why it's ungrammatical, yes A and B can stand for noun phrases but not necessarily, when I say a mammal you see any mammal that's available when I say mammals you see mammals as a whole, both of which "A", a cat is a part of


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## heypresto

A cat is a type of mammal.   
All cats are mammals. 

A mammal is _not_ a type of cat. 

A mammal is a cat. 
All mammals are cats. 

A cat is a mammal, but not all mammals are cats. 

_Some _mammals are cats. 

Some mammals are cats, some are dogs, some are cows, some are whales, some are bears, some are humans . . . etc. 



Several mammals (not cats) have tried to answer your question. 



Is it the logic you don't understand, or the words we use when making these logical arguments and statements?


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## Keith Bradford

ans7 said:


> ...But I want to use "a/an", the whole problem is that I can't use a and an


Are you complaining that your table is ugly and malfunctional because you *wanted *to use a hammer when I've told you in #19 that experts use a screwdriver?  If you *want *to use an inadequate word, you only have yourself to blame when the legs fall off, sorry.


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## ans7

Keith Bradford said:


> Are you complaining that your table is ugly and malfunctional because you *wanted *to use a hammer when I've told you in #19 that experts use a screwdriver?  If you *want *to use an inadequate word, you only have yourself to blame when the legs fall off, sorry.


Using all and some is okay but half of the times we have to use a and an, I can use a screwdriver anytime I want but I made this whole thread to discuss how we fix tables with a hammer


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## heypresto

ans7 said:


> Using all and some is okay but half of the times we have to use a and an


If you understand '_all_ cats are mammals' and '_not all_ mammals are cats', and '_some_ mammals are cats', what is it about 'a cat is a mammal, but a mammal is not a cat' that you don't understand?


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## Uncle Jack

ans7 said:


> what i don’t understand is how do one say something about the subject? what is “a mammal” to “a cat”?


One of its attributes or properties.

A cat (which in the original sentence I understood to be a representative member of the species _Felis catus_, but perhaps you intended "a cat" to have a different meaning) is many things:
A cat is a mammal.​A cat is an animal​A cat is a quadruped.​A cat is carnivorous.​A cat is covered in fur.​A cat is smaller than a horse.​​


ans7 said:


> Well the problem is that there are times when A isn't a subset of B.
> 
> My dad is the man
> The man is my dad


This is right. One use of "be" is to say that the subject (a cat) is of a type described by the complement (a mammal), but this is just one use of "be".

Subjects fall into different categories, and different categories of subject can have different types of complement. As I said earlier "a cat" in your original sentence refers to a representative example of a cat. We know this because the original sentence "Why is a cat a mammal but a mammal isn't a cat?" makes no sense if "a cat" refers to an individual cat, which is the only other possibility that I can see. However "my dad" and "the man" in your two new sentences both describe an individual male human being. Where the subject is a unique individual, then other complements are possible with "be", including the complement also being a unique individual. When this happens, it is possible to exchange the subject and the complement.
My dad is the oldest person in our street.​The oldest person in our street is my dad.​ 
The same thing is sometimes possible where the subject and complement are both a unique group, but it becomes unlikely where the subject is an abstract noun, a class of thing or a general plural, and I think it is impossible where the subject is a representative example.



ans7 said:


> And then there's the articles: a/an
> A mammal is a cat OR a tiger OR a human OR...........
> This basically means a mammal is a cat ight? That's what the conjunction "or" does





ans7 said:


> Is "a" mammal a group?


This is a good question. "A mammal" can mean an example of the biological class Mammalia, or it could mean an individual being belonging to the biological class Mammalia, and I am not sure which interpretation is best in "A cat is a mammal". However, when "a mammal" becomes the subject in a clause dealing with generalities, we usually take "a mammal" to refer to a representative example of the biological class Mammalia, which is a different thing from merely being an example of the class. "A mammal" as the subject of a clause can also refer to an individual animal: Look! There's a mammal. We can only tell these meanings apart by looking at the context and seeing what alternative meanings fit.

It might be worth noting that when "a" + [singular noun] refers to a representative example of that noun, it is almost always possible to replace it with [plural noun] (without an article), to refer to the group of things represented by that noun as a whole. This is not possible with other meanings of "a" + [singular noun].

However, if you change the subject from singular to plural, you also need to change the complement:
A cat is a mammal.​A representative member of the species _Felis catus_ is an example of the biological class Mammalia, or is an individual being belonging to the biological class Mammalia.​​Cats are mammals.​Members of the species _Felis catus_ as a whole are examples of the biological class Mammalia, or are individuals being belonging to the biological class Mammalia.​
I am not sure if this helps.


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## dojibear

ans7 said:


> now I just want to know how "be" works


You want human language defined in terms of logic and mathematics. That might not be possible.

"Be" is used in several different ways, not just one way. "Be" does not "work" in only one way.



ans7 said:


> Using "all" and "some" is okay but half of the times we have to use "a" and "an".


No, we don't. We never have to use "a" and "an" when describing logical rules in English.



ans7 said:


> the whole problem is that I can't use "a" and "an"


The problem is that you can't use them *yet*.

Part of the problem is that you switch between "a/an" and "be", which are two totally different topics. No wonder the thread is confused! How can we answer, when you keep changing the question? At one point you said "the man", pretending that "a" and "the" are identical.

I have found that English "a/an" often matches "one" in other languages (Mandarin Chinese, for example).
Google Translate says that "a" in "a boy" (in Vietnamese) is _Một_.

The various uses of "be" in English are expressed in different ways in different languages.



ans7 said:


> It's because of languages that this is a problem, more specifically because of the linking verb "be", the verb itself creates a whole new logic that shouldn't exist


Who says it shouldn't? What gives you the right to decide how other people talk?

"Be" is used in several different ways in English. Each use has its own "logic". It isn't possible to lump them all together. There is no "logic" for all uses of "be". The verb does not create a whole new logic.


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## elroy

I already offered you, many posts ago, a way to start with “a mammal” and create a meaningful sentence:


ans7 said:


> what is “a mammal” to “a cat”?





elroy said:


> A mammal is what a cat is.





ans7 said:


> thanks that really helps yeah



I thought that settled it.  I can’t believe this thread has gone on for this long.


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## suzi br

ans7 said:


> Yes I know we need the articles but why is that? Don't the rules say if A is a part of B, A is B? A cat is definitely a part of mammals



Why do you say “yes I know” but ignore the information?


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## ans7

Uncle Jack said:


> One of its attributes or properties.
> 
> A cat (which in the original sentence I understood to be a representative member of the species _Felis catus_, but perhaps you intended "a cat" to have a different meaning) is many things:
> A cat is a mammal.​A cat is an animal​A cat is a quadruped.​A cat is carnivorous.​A cat is covered in fur.​A cat is smaller than a horse.​​
> This is right. One use of "be" is to say that the subject (a cat) is of a type described by the complement (a mammal), but this is just one use of "be".
> 
> Subjects fall into different categories, and different categories of subject can have different types of complement. As I said earlier "a cat" in your original sentence refers to a representative example of a cat. We know this because the original sentence "Why is a cat a mammal but a mammal isn't a cat?" makes no sense if "a cat" refers to an individual cat, which is the only other possibility that I can see. However "my dad" and "the man" in your two new sentences both describe an individual male human being. Where the subject is a unique individual, then other complements are possible with "be", including the complement also being a unique individual. When this happens, it is possible to exchange the subject and the complement.
> My dad is the oldest person in our street.​The oldest person in our street is my dad.​
> The same thing is sometimes possible where the subject and complement are both a unique group, but it becomes unlikely where the subject is an abstract noun, a class of thing or a general plural, and I think it is impossible where the subject is a representative example.
> 
> 
> 
> This is a good question. "A mammal" can mean an example of the biological class Mammalia, or it could mean an individual being belonging to the biological class Mammalia, and I am not sure which interpretation is best in "A cat is a mammal". However, when "a mammal" becomes the subject in a clause dealing with generalities, we usually take "a mammal" to refer to a representative example of the biological class Mammalia, which is a different thing from merely being an example of the class. "A mammal" as the subject of a clause can also refer to an individual animal: Look! There's a mammal. We can only tell these meanings apart by looking at the context and seeing what alternative meanings fit.
> 
> It might be worth noting that when "a" + [singular noun] refers to a representative example of that noun, it is almost always possible to replace it with [plural noun] (without an article), to refer to the group of things represented by that noun as a whole. This is not possible with other meanings of "a" + [singular noun].
> 
> However, if you change the subject from singular to plural, you also need to change the complement:
> A cat is a mammal.​A representative member of the species _Felis catus_ is an example of the biological class Mammalia, or is an individual being belonging to the biological class Mammalia.​​Cats are mammals.​Members of the species _Felis catus_ as a whole are examples of the biological class Mammalia, or are individuals being belonging to the biological class Mammalia.​
> I am not sure if this helps.


Well thanks for the answer


dojibear said:


> You want human language defined in terms of logic and mathematics. That might not be possible.
> 
> "Be" is used in several different ways, not just one way. "Be" does not "work" in only one way.
> 
> 
> No, we don't. We never have to use "a" and "an" when describing logical rules in English.
> 
> 
> The problem is that you can't use them *yet*.
> 
> Part of the problem is that you switch between "a/an" and "be", which are two totally different topics. No wonder the thread is confused! How can we answer, when you keep changing the question? At one point you said "the man", pretending that "a" and "the" are identical.
> 
> I have found that English "a/an" often matches "one" in other languages (Mandarin Chinese, for example).
> Google Translate says that "a" in "a boy" (in Vietnamese) is _Một_.
> 
> The various uses of "be" in English are expressed in different ways in different languages.
> 
> 
> Who says it shouldn't? What gives you the right to decide how other people talk?
> 
> "Be" is used in several different ways in English. Each use has its own "logic". It isn't possible to lump them all together. There is no "logic" for all uses of "be". The verb does not create a whole new logic.


>Be" is used in several different ways, not just one way. "Be" does not "work" in only one way.

 Yes, the other ways are easy to learn because it follow a set of logic, see #34.

The last way happens when you use the articles:a/an or none at all for the objects

>"No, we don't. We never have to use "a" and "an" when describing logical rules in English.

I'm not saying we use them to describe logical rules in English, I'm talking about times when we just use them in general, have you never used a/an in a sentence your entire life?

>Part of the problem is that you switch between "a/an" and "be", which are two totally different topics. No wonder the thread is confused! How can we answer, when you keep changing the question? At one point you said "the man", pretending that "a" and "the" are identical.

The only way the solve what "be" is is to dive deeper into its functions and laws, that requires examples from the complements, in this case: a/an


>I have found that English "a/an" often matches "one" in other languages (Mandarin Chinese, for example).
Google Translate says that "a" in "a boy" (in Vietnamese) is Một.

That's the problem, một means one, not a/an.

>Who says it shouldn't? What gives you the right to decide how other people talk?

I'm not deciding anything, people can talk however I want, I won't judge but there's a flaw in their languages

>"Be" is used in several different ways in English. Each use has its own "logic". It isn't possible to lump them all together. There is no "logic" for all uses of "be". The verb does not create a whole new logic.

Yes, there are many ways to use "be", all but one of which I've understood, and it's because of the logic in this one that I can't comprehend


elroy said:


> I already offered you, many posts ago, a way to start with “a mammal” and create a meaningful sentence:
> 
> 
> 
> 
> I thought that settled it.  I can’t believe this thread has gone on for this long.


Sorry I was being sarcastic


suzi br said:


> Why do you say “yes I know” but ignore the information?


I didn't ignore it though, saying something is right and wrong doesn't add any logic to it, the information given doesn't follow the set of rules given









I think, although flawed and illogical, what's happening in these clauses:
-Cats are mammals
-A cat is a mammal
_is the result of "to be" being used for _classification _not _equality, _this sort of classification is different in the fact that they can be classified singularly or plurally.

-These men are a group

In this case, the men belong in the class: "group" singularly


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## Uncle Jack

ans7 said:


> I think, although flawed and illogical, what's happening in these clauses:
> -Cats are mammals
> -A cat is a mammal
> _is the result of "to be" being used for _classification _not _equality, _this sort of classification is different in the fact that they can be classified singularly or plurally.


"Be" is rarely used for equality, as I had hoped to make clear in post #43. I am not sure that I would say that "be" is used for classification here, but perhaps this a good way of looking at it. This, too, is a relatively unusual use of "be".



ans7 said:


> -These men are a group
> 
> In this case, the men belong in the class: "group" singularly


In this sentence, we interpret "be" as meaning something like "comprise" or "form". "Group" is a concept rather than a classification, and each group needs to be defined. This particular group is defined in your sentence: it consists of these men. This makes this use of "be" one of equivalence, and you can therefore say:
These men are a group.​The group is these men.​Note that the second sentence requires the group to have already been introduced (and so the definite article is used). You cannot usually use this form of "be" to introduce something specific as the subject, other than in the form "there is a group". This is one reason why when we read "a cat is a mammal" we immediately interpret "a cat" to be a general subject, a representative member of the species.


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## Glasguensis

I take it you’re aware that there is no need for grammar to be logical, or for it to be easy or even possible to express it in terms of rules?

We have seen many people having problems with article use in English : it is easy to outline some basic principles but this doesn’t cover every situation. Likewise with the verb « to be ». 

The structure « a x is a y » can mean *either* « x equals y » *or* « x belongs to the category y ». The grammar alone doesn’t tell us which: like so much else in English it’s the context which tells us.


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