Singular vs Plural – ‘I Don’t Like a Dog Who Barks’ or ‘I Don’t Like Barking Dogs in General’?

When you give a definition for a word, don't you usually say something like this

A cat is a four-legged animal with ...

or this?

Schizophrenic means ...

I don't think you should drop the article when you're going for the first type of response. That way, you're teaching the two-year-old about grammar rules while you're at it.

I don't think it matters whether you're referring to a specific cat or not, because "Cat is a four-legged animal ..." is not grammatically correct. You either say "A cat" or "The cat", else you say "Cats are ..."

In any case, when you showed your imaginary son the picture of a cat and a dog, it didn't matter that you weren't talking about that specific cat or dog, because at that point, you were referring to a specific cat and dog (the ones in the pictures).

Interestingly, you can say "a bible" if you're referring to one of many physical, printed versions of the book.

I have two bibles in my house, a bible from my grandmother's family, and a bible from my grandfather's.

Why is there a Bible in every hotel room?

When you say "the Bible," however, you're referring to the literary work itself, rather than a specific instance or printed version of the work. I think we simply think of it as a proper noun, as though "the" is part of the title.

Looking into this I did find this discussion, which may be of interest. It didn't seem to come to a consensus as to why this is grammatically, but some users mentioned the etymology of the word as likely contributing to this usage. "Bible" comes Latin biblia meaning simply "book." Essentially, people used to simply refer to it as "the book," and I suppose the definite article just stuck around while "Bible" evolved into a word of its own.

Worth noting is that there are a lot of historical literary works whose names use the, including religious texts (The Torah, The Qur'an) and epic poems (The Iliad, The Aeneid).