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 mean "in this way"?
See entry #2. com/dictionary/american/so_1
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See entry #2. "so" is a repetition of the phrase "what pushed that theory forward"
https://www.macmillandictionary.com/dictionary/american/so_1
XVII don't know the use of the word "so" in the text. Does it mean "in this way"?
No. It means something like "as". What Engles wrote in his book Ludwig Feuerbach and the End of Classical German Philosophy is the primary source for the opinion the writer propounds in the sentence, that progress pushed the materialistic theory forward. To paraphrase: