Why "it’s turtles" not "they are turtles"

Question

It is a third person singular and is used to refer to a thing. If that’s the case, then why do we say:

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on.” “You’re very clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”
—from A Brief History of Time by Stephen Hawking

I would use it instead of they because the former sounds natural to me, but I wouldn’t be able to explain the convoluted details (unless they are simple) behind it.

Answer 1

This particular it is a Dummy Subject pronoun, Distance it; the construction requires a locative of some sort and estimates the extent of some stretch of (perhaps metaphorical) landscape.

• It's 31 miles as the crow flies from Bellingham to Mt. Baker.
• It's a long way to Tipperary.
• It's just corn out there, as far as the eye can see.

In the quoted sentence (quite a famous one in linguistics, because Haj Ross used it as the Fragestellung (pp iv-v) of his extremely influential dissertation, though he blames it on William James, not Bertrand Russell)

• It's turtles all the way down.

it doesn't refer to turtles, but rather — if it can be said to have any reference — to the "landscape" metaphorized by the old lady's comments (interestingly, it's always a little old lady in the story, though the prominent intellectual varies).

That is, we are seeing, through her imagination, turtles stacked one atop another below us, for as far as we can see (using see metaphorically, of course, but that's normal for imaginary landscapes).

Answer 2

The antecedent of “it” is “what the tortoise is standing on”. The sentence is equivalent to:

You’re very clever, young man, very clever, but what the tortoise is standing on is turtles, all the way down.

Answer 3

It's here is rather like there are. It doesn't have an antecedent, but acts as a subject where there otherwise wouldn't be one.

Answer 4

Even though I'm not able to describe this in grammatical terms, I'll try to give you a "feeling".

We don't refer to the turtles themselves, so we're not interested in their numbers. We're interested just in the fact that there are turtles, in other words the concept of turtles. You could say "But, there are turtles all the way down!", but not they.

"It's" could be replaced with "the fact is that there".

Answer 5

I think this particular it is the "existential it".

In this context, where they're effectively talking about the "universal backdrop" within/above which our world exists, it seems to me the same as the "everything, but nothing in particular" referrenced when we say "It's raining", or "It's always women {who do something}".

Answer 6

I concur with those who link the it to the support structure introduced in the question.

I disagree with the view that "it is" would be more generalized than "there are", since "there are" links to the turtles, not the support structure, without referencing how those turtles are arranged.

I believe the old woman intended to conjure a specific visual of an endless stack of turtles, one atop the other.

In that case, using it to reference the question about immediate support under the first turtle is necessary in order to create the arrangement of a turtle standing on another turtle and and so forth.

If one wishes not to be impose any architectural design to the supporting turtles, "there are" would be the ticket. That frees the turtles from a stacked reference and improves conjuring what one might wish, say, a messy pile of turtles.

There are meaningful nuanced differences in language that may not be covered in specific rules.

Answer 7

My interpretation is that "it" is "the [singular] structure that supports the world," and it is made of turtles.