The following appears in a 2007 article in Constant-Content:
“If, then” statements require commas to separate the two clauses that
result.
If I use correct punctuation, then I will include commas where necessary.
If Hillary Clinton wins the election in 2008, then she will become the first female president.
[If enough was known about a thing to distinguish it from all other things, then all its properties could be inferred by logic.]
Even when the statement drops the word then, a comma must be used.
If Santa Clause was real, he would bring me a new bike.
If I had the money, I would go to Krakow for vacation.
Even the minimalists advocating the use of fewer commas would rarely omit one after a protasis (though there are examples of the terse, dramatic If he fights he dies on the internet, and I personally find this an acceptable choice).
However, the addition of a report tag before the protasis changes the cadence of the whole sentence. I'd say that this licenses the dropping of the comma after 'things', allowing
- [1] Hegel thought that if enough was known about a thing to distinguish it from all other things then all its properties could be inferred by logic.
This is a possible reading (for those with good voice control). It also removes a problem some could see of a comma after the 'that' of a report tag. Omitting the 'then', however, virtually forces the comma after 'things' to aid parsing, so I'd not omit it here.
- [2] ??Hegel thought that, if enough was known about a thing to distinguish it from all other things then all its properties could be inferred by logic.
looks very clumsy.
- [3] Hegel thought that, if enough was known about a thing to distinguish it from all other things, [then] all its properties could be inferred by logic.
both look more acceptable, and again represent one way of reading the sentence. As Kate says, this makes a lengthy sentence easier to parse (especially if 'then' is retained in the well-known if ...then construction) and to read aloud. Perhaps it looks more acceptable because the structure precisely
mimics a parenthetical structure (though the protasis is vital).