"In most written English the paragraph is of crucial importance, since that which the writer is trying to say is essentially conveyed by a good ordering of paragraphs."
Can this since-clause be paraphrased as "since what the writer is trying to say is..."? Is there any difference in the meaning of the two? Which would you prefer? My version seems to me more elegant but am not that sure of it.
Re: That which
Your version is fine. 'That which' is more likely to be found in formal language; colloquially most would use 'what'.
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