Questions about “The Gettysburg Address”

Question

It is for us the living, RATHER(should here be a comma or no comma) to be dedicated here to the unfinished work which they who fought here have THUS FAR so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us -- that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion.....

The above is a excerption of the Gettysburg address, so here's my two questions:
1. As I mentioned above, should it be with or without a comma after the word "rather". Actually, I searched on the Internet, it has different versions, some have a comma after rather, some don't.
2. The second one is, can the phrase "thus far" be replaced by "so far", if it could, doesn it mean that these two are totally interchangeable; if not, where's the difference?

Answer 1

Your instinct on the comma is quite right. Here it is being used to mark out a parenthetical remark ("rather"), a short aside that could just as easily be put in brackets as I did there.

You need to be wary of being technically correct with commas, though. If you stick a comma absolutely everywhere that one could go, your text may look like it has a severe case of measles. In many cases, commas are optional. I wouldn't normally omit parenthetical commas like these, but if I did I would drop both of them, not just one.

"Thus far" can be replaced with "So far", but there are two good reasons why you shouldn't in this case. First, "thus far" is a more formal phrasing, and fits better in the formal context of the Gettysburg Address. "So far" is more neutral in that respect; it isn't wrong, but "thus far" is better.

The second reason not to use "so far" here is that it would repeat the word too quickly, drawing attention to it in a way that distracts from the speech.

"[...] which they who fought here have so far so nobly advanced."

These are two different, unrelated uses of the word "so", and putting them close together makes them seem more related than they really are.

Answer 2

Let me list the usages of a comma:
It is used to separate items in a series i.e. pears, fruits, and strawberries.
It is used to separate clauses and phrases in a sentence i.e. "He took a bite, and ran all the way home."
It is used before quotes i.e. Mr. John said, "Hie thee home!"
It is used in between adjectives i.e. The dull, red river.
It is used in numbers i.e. 1,000,000
It is used in dates i.e. 7th of May, 1978

However, the thing that might have confused you, was the usage of the comma to set off certain adverbs, like "Therefore", "however", etc.

Although "rather" is an adverb, it is not one of the adverbs that are set off by commas when in a sentence.

For your second question, Yes, "thus far" can be replaced by "so far", as can be seen here:

so far, hitherto.