How does Ego know what a celestial is?

Question

Ego says he is a celestial, and tells his story as finding himself alone in the universe, and searching for other life. When he finds that life, he calls it "disappointing", which would seem to indicate it was not life of his species.

But then how does he know what Celestials are? How does he know he is one? He clearly seems to think there are others like him, since he says he is a celestial, not the celestial or anything similar.

For purposes of this question, let's set aside the fact that Ego isn't a celestial in the 616 universe; we're just talking about the cinematic universe. Let's also assume that Ego wasn't purposely lying or otherwise misleading Peter (unless we have strong evidence that he was).

Answer 1

How does anyone know what a Celestial is?

In the first film, Gamora knows that Knowhere is the severed head of an ancient celestial being.

It's called Knowhere. The severed head of an ancient celestial being.

Knowhere has been inhabited by a lot of people for centuries at least, so this must be common knowledge, and the concept of a "celestial being" must be common knowledge too, or that statement would have no meaning.

The Collector also shows the Guardians an image of a celestial using the stone in their possession to destroy a planet.

These stones, it seems, can only be brandished by beings of extraordinary strength. Observe. [he shows them the giant beings that use the Infinity Stones as weapons] These carriers can use the stone to mow down entire civilizations like wheat in a field.

I think there is also a mural of a celestial behind the Grandmaster's box seats in the trailer to Thor: Ragnarok, though this may just be an easter egg.

So celestials are common knowledge, though we are never told how.

Guardians of the Galaxy 2 spoiler warning.

Ego was originally alone for a very long time, but by the time he meets Peter and tells his story he is very familiar (wink wink, nudge nudge) with life throughout the universe. He has already visited and knocked up women on thousands of planets! If everyone in the galaxy knows about celestials, then the incredibly cosmopolitan Ego is certain to know enough about them to have guessed that he is one too.

Answer 2

Ego is a confirmed Celestial

Ego may have been a liar about many things, but not about being a Celestial. As confirmed by Marvel Studios’ The Marvel Cinematic Universe: An Official Timeline, a book that gives a chronological account of the events of the MCU, Ego is a Celestial, and he and his fellow Celestials are older than the universe itself, and even predate the Infinity Stones.

Ego’s encounters with his own kind

So, how does Ego know what a Celestial is? Well, he has lived for eons, traveled across the universe, and witnessed many things. And as we discover in the Eternals film, it is the Celestials that create suns, shape galaxies, and spread life. Ego is very likely to have run into some of his own kind, or at least learned of them, in his travels. It can be assumed that he has a pretty good idea of what a Celestial is.

Ego and the Celestials were the first characters introduced in the official timeline:

Dawn of the Universe | The Handiwork of Celestials

Among the entities who shape the early universe are the Celestials, one of the oldest and most powerful forms of life to ever exist. These god-like beings pre-date the birth of the cosmos as well as the Infinity Stones, and use their ability to manipulate matter and energy to help construct suns that make life possible on other worlds.

Dawn of the Universe | Ego Takes Shape

Not all Celestials are born from the cores of planets. Some evolve seemingly by happenstance, like the Celestial known as Ego. At first just a spark of consciousness alone in deep space, Ego manipulates matter to construct physical neural-networks for itself in a large, brain-like receptacle, then assembles an entire planet layer by layer around that. While other Celestials exist in the form of colossal giants, Ego prefers to mimic more humble forms of life, creating avatars that look like "lesser" sentient creatures found on distant worlds.

Answer 3

Philosopher here, will offer a philosophy of language, general response. Hopefully it's useful, as it approaches the question from a totally different angle (and can be taken together with the chosen answer).

The way your question is phrased suggests you are thinking in terms of the "museum theory" of language: the thought that the world is like a museum exhibit, and we have a sack of labels (words), that correspond to each item. The bag taken as a whole represents language, and as we go about the museum, we affix the correct label to its item.

In this case, your question implies that there is a label "Celestial" in the bag (a word floating in "language-space," if you will) that must be affixed to Ego, because that is what he is. But language doesn't work that way: Ego is whatever he is, and it's the community of language users that decide (sometimes deliberately, other times through chance and accidents of history) what to call him or anything else, for that matter (and this can change over time). He can also determine that himself, and just spread the word. So, his powers, abilities, etc., would remain the same, regardless if he were called a Celestial or a Pzun (and then you would be asking "how did Ego know what a Pzun is?")

Consider that the word "human" appeared in English around the 1530s: how did these first users of the word know what a "human" is? What were they before realizing? Obviously they were exactly the same as before the word came into the language, and just used a different word (then you could ask "how did they know that was the correct word?" Again, there isn't a "correct" word: there are just strings of sounds that we attach to things for expediency and convenience).

So, in a way, no one knows what a Celestial (or a human or a Pzun) is: rather, speakers decide (or are told, by Ego or his former peers) to call a certain thing a Celestial.

I'm oversimplifying/leaving out details, but hope it makes sense.