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A cat is an animal.

Cats are animals.

Do the two sentences mean exactly the same thing? If not, what are the differences and how do you use them?

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    They are the same. However, 'a man is an animal' and 'men are animals' can have different meanings. :-)
    – mcalex
    Commented Aug 6, 2021 at 7:39

5 Answers 5

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A cat is an animal.
Cats are animals.

The meaning of these two statements is, essentially, the same.

Of course, "a cat" is singular, and "cats" is plural. In most contexts, "a cat" would refer to just one cat. However, in the statement "a cat is an animal", a single cat is being used as an example that is representative of all cats, and that's why it means the same as the second statement.

It is quite common to use a single example as representative of the group to which they belong. A couple of well-known sayings about dogs are examples of this:

  • A dog is a man's best friend.

Although referring to "a dog" and "a man", this idiom suggests that all dogs are good companions for people.

  • A dog is for life, not just for Christmas

This was a slogan from animal welfare groups in the UK to warn people against buying pets as Christmas presents, as many are abandoned soon after. Again, "a dog" represents all dogs.

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  • The singular in "dog...best friend" seems purposeful. We're saying that each man has one particular dog as a best friend. Compare to the plural "Diamonds are a girl's best friend" which implies a woman would be happier with lots of diamonds of all types. Likewise "a dog is for life..." is conjuring an image of the one particular dog you're thinking about as a present. Commented Aug 5, 2021 at 19:11
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    @OwenReynolds I'd have to completely disagree with you. Some people own and love several dogs. Other people have no dogs. It's a generalisation, that any dog could be a great friend for any person. Not everybody agrees with generalisations. Regarding "diamonds...." notice it is "a girl's best friend". The girl is singular, but you don't think it's talking about just one girl, do you? The inference there is that the average girl wants more than one diamond.
    – Astralbee
    Commented Aug 5, 2021 at 20:50
  • @Astralbee yes, and to follow along with that logic and refute Owen Reynolds’ point, the average girl wants more than one diamond, the average man wants a dog. Commented Aug 6, 2021 at 0:34
  • There actually is a difference. "A dog is a man's best friend" posits a norm, which many have exceptions; "all dogs are their respective man's best friend" posits that there are no exception.
    – toolforger
    Commented Aug 6, 2021 at 6:35
  • Now we know that all cats are animals, and interpret "a cat is an animal" as meaning the same thing - i.e. it's our background knowledge that makes us accept the phrases as equivalent, not the language, since the meaning does vary for the "a dog is a man's best friend".
    – toolforger
    Commented Aug 6, 2021 at 6:37
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Logically speaking they are equivalent. A cat is an animal if and only if cats are animals, so both sentences can be used interchangeably.

As you noticed yourself the difference is that grammatically speaking one is singular and the other is plural. This only matters if you combine the clause with another one and they have to agree on plurality. Take the following examples:

A. A cat is an animal and it needs to eat.
B. A cat is an animal and they need to eat.
C. Cats are animals and it needs to eat.
D. Cats are animals and they need to eat.

Here, the meaning of it and they changes depending on which clause is put in front.

In case A, the it means that cat (and implies it needs to eat because all animals need to eat).
In case B the they typically means animals (and given that the cat is an animal we can derive that the cat also needs to eat).
Case C is wrong. The it being singular wants to point to a specific singular "thing" in the first clause but there are only plurals in there so it doesn't really work, though nobody will have a problem understanding what you mean anyway.
In case D it's unclear whether the they points to the cats or animals in general (but both work so it doesn't really matter).

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    Great reply! I didn't knew we can add singular and plural clauses as in example B. Commented Aug 5, 2021 at 11:22
  • I disagree a bit with you first paragraph. I get what you're saying, but I don't think the appeal to logic works. Purely logically, "A cat is an animal" could be interpreted as existential ("There is some cat that is an animal.") or universal ("For any given cat, it is an animal.") It's purely a matter of English semantics that we mean the latter and not the former. Commented Aug 6, 2021 at 2:02
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    @SyedDanishAnwar To nitpick: the singular/plural number does not attach to the whole clause, just to the pronoun. Unpacking the pronoun: "A cat is an animal and animals need to eat". Compare "Ross is a human and humans do not have six legs." (Which might be the response to someone asking "Why doesn't Ross buy shoes in sets of six instead of two?") Commented Aug 7, 2021 at 17:46
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In most contexts "a cat" and "cats" have different meanings. "Cats" is plural and refers to multiple individuals.

In this particular context, the meanings of the two sentences are pretty much identical. You would use use the first if someone one asked you

What is a cat?

and the second if asked

What are cats?

And you would ask the first question if you were reading something and found the words "a cat" and didn't know what it meant, and the second if you found the word "cats".

It would be pretty rare to actually ask this about "cats" as that is a very common word. It would be rare to use those particular answers, since they don't really help define the word.

You can find examples on this site:

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    Although I agree that we would typically answer the way you describe, but it's not wrong to just swap them either. What is a cat? Cats are animals that purr. What are cats? A cat is an animal that purrs.
    – Imus
    Commented Aug 5, 2021 at 7:34
  • You did make me think of a case where it does matter: If you are asking about something where there is only one of. For example: What is a president? Answering that with "presidents are ..." could imply that there is more than one president within a country. (it's possible that another country has it's own president so multiple presidents do exist).
    – Imus
    Commented Aug 5, 2021 at 7:44
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    @imus Hmmm. "Presidents are wise, strong leaders..." feels fine. it describes all past and future presidents. So does "Presidents are housed in the White House". But "Presidents are going to be in Nebraska next week" definitely implies more than one. Commented Aug 5, 2021 at 19:19
  • @OwenReynolds I agree your sentence does describe all past and future presidents, but that description may or may not be accurate in individual cases... Commented Aug 7, 2021 at 17:48
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They are more or less the same, although it can be argued that the word "cats" could cover not only housecats (companion cats), but also other species of felines, such as lions, tigers, etc. Still, they would be also animals, so it's a nuance.

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Yes, they mean approximately the same thing.

When we make general statements in English, we can frequently use either a singular or a plural subject.

If we use a singular subject, we interpret the subject as a representative or ideal of the class we're discussing. So in, “A cat is an animal,” the cat in question is some nonexistent ideal cat, having all the qualities of a cat and nothing that is not cat. So if that cat is an animal, all cats are animals.

If we use a plural subject, we're talking about the class itself. So in, “Cats are animals,” we're making a direct statement about all cats. Implicitly, we're saying, “All cats are animals.”

(Most English speakers hearing these sentences wouldn't consciously analyze them like this. They might not even notice which form was used. I just thought it might be helpful to see why the two sentences wind up meaning the same thing.)

Use whichever form makes the sentence easier to follow.

Especially if the phrase is embedded in a larger sentence. For example,

The child knows that a cat is an animal, and it likes milk.

A bit unclear. Does the cat or the child like milk?

The child knows that a cat is an animal, and it, the cat, likes milk.

Awkward, but clearer. We're using an explicit appositive to make it clear we're referring to the cat. I might say this if I were speaking aloud and realized too late I was being ambiguous.

The child knows that cats are animals, and they like milk.”

Here “child” is singular and “cats” is plural, so it's easier to follow that cats also like milk, no explanation required.

That said, if it's clear enough from context who's who, you can usually use whichever style you prefer. Even in my example, it would probably be fine to use the first sentence — most people know that cats like milk, and it would be odd to give those two facts about the child as one sentence. The listener may just have to think a moment more.

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