the use of the word "so"

Question

This determined at once the strength and the weakness, the achievement and the limitation, of the materialist theory.

What pushed that theory forward was, so Engels writes in Ludwig Feuerbach, "the powerful and ever more rapidly onrushing progress of science and industry".

Materialism and the Dialectical Method (Maurice Cornforth)

I don't know the use of the word "so" in the text. Does it the same as the word "as"?

Answer 1

It's similar to saying "as," but it gives more weight. The author is basically attributing this point to Engels' writing, like saying "Engels said as much in his writing..."

Answer 2

"So" means it is claimed that Engels wrote that statement in the Ludwig Feuerbach, but there is no way to prove it.

If we say "so she said, so he wrote, etc. It means that someone else is telling something but they can't confirm it really happened. They can't confirm she said something or he wrote something so we don't know if it is true.

Example: It's going to rain, so he says, but we'll take our umbrella just in case.

Hope that helps.