So one of the interesting nuances is that it isn’t talking about the Platonic forms. If it was, it would have used eidos.

The text is very much engaging with the Epicurean views of humanity. The Epicureans said that there was no intelligent design and that we have minds that depend on bodies so when the body dies so too will the mind. They go as far as saying that the cosmos itself is like a body that will one day die.

The Gospel of Thomas talks a lot about these ideas. For example, in saying 56 it says the cosmos is like an already dead body. Which fits with its claims about nonlinear time in 19, 51, and 113 where the end is in the beginning or where the future world to come has already happened or where the kingdom is already present. In sayings 112, 87, and 29 it laments a soul or mind that depends on a body.

It can be useful to look at adjacent sayings, as the numbering is arbitrary from scholars when it was first discovered and they still thought it was Gnostic instead of proto-Gnostic.

For 84, the preceding saying is also employing eikon in talking about how the simulacra visible to people is made up of light but the simulacra of the one creating them is itself hidden.

This seems to be consistent with the other two places the word is used.

In 50, it talks about how light came into being and self-established, appearing as “their simulacra” (which is a kind of weird saying as who are they that their simulacra existed when the light came into being - this is likely why the group following the text claim their creator entity postdates an original Adam).

And in 22 it talks about - as babies - entering a place where there’s a hand in place of a hand, foot in place of a foot, and simulacra in place of a simulacra.

So it’s actually a very neat rebuttal to the Epicureans. It essentially agrees that maybe there isn’t intelligent design like they say and the spirit just eventually arose from flesh (saying 29), and that the cosmos is like a body, and that everything might die. But then it claims that all that already happened, and that even though we think we’re minds that depend on bodies, that we’re the simulacra - the copies - not the originals. And that the simulacra are made of light, not flesh. And we were born into a simulacra cosmos as simulacra people.

From its perspective, compared to the Epicurean surety of the death of a mind that depends on a body, this is preferable. Which is why you see it congratulate being a copy in 18-19a:

The disciples said to Jesus, “Tell us, how will our end come?”

Jesus said, "Have you found the beginning, then, that you are looking for the end? You see, the end will be where the beginning is.

Congratulations to the one who stands at the beginning: that one will know the end and will not taste death."

Jesus said, "Congratulations to the one who came into being before coming into being.

The text employs Plato’s concepts of eikon/simulacra to avoid the Epicurean notions of death by claiming that the mind will live again as a copy and we are that copy, even if the body is screwed. This is probably the central debate between this sect and the canonical tradition. The cannonical one is all about the body. There’s even a Eucharist tradition around believers consuming Jesus’s body to join in his bodily resurrection. Thomas has a very different Eucharistic consumption in saying 108, where it is not about drinking someone’s blood but about drinking their words that enables becoming like someone.

It’s a very unusual philosophy for the time. Parts of it are found elsewhere, but the way it weaves those parts together across related sayings really seems unique.