Henri, I have decided, since I turned 71 on Wednesday, to stop pussy-footing around on these issues on the several listservs I am on. This whole issue has been thoroughly researched, studied, and explored for about 100 years now. I myself have about 20 books on my shelves which explain this historically, linguistically, literarily, and philologically. There is absolutely no reason for anyone to “opine” on this issue. I will be happy to recommend books of the highest caliber, nothing marginal, new, untraditional, new-fangled, or controversial. Also, I think we sometimes set up straw men (usually in the form of 5th grade English teachers) who, we claim, enunciate and enforce these bogus rules; in fact, I have checked grammar manuals for schools written 60+ years ago, even in the 1930s, and they are very careful about recommending some of these rules which go against normal English speech. Most genuine experts on English usage recognize both the range of good usage and the historical roots of that usage (as well as the well-defined history of these bogus rules most attributable to Bishop Lowth of London and Lindley Murray, a textbook publisher, both writing in the 1700s; many prominent scientists and scholars of the time, like Joseph Priestly, strenuously objected to these “rules” being invented, but school teachers were so eager for authoritative/authoritarian textbooks, they inundated students with the books and their rules).
The real issue is a deep philosophical one that has been with us for centuries, going back to the Greeks: do we observe and describe or do we establish premises and then rationalize them? If someone wishes to follow a rule about not “splitting infinitives”, they are perfectly OK to do that (although I don’t believe they should teach it since it has no factual basis). In order to justify the rule, they can come up with some sort of reasoning, such as, “you should not split an infinitive in English because the infinitive is a single grammatical unit with its own part of speech.” And if that satisfies someone, then that’s fine. Linguists have tests for such rules, but many people reject linguistics, and so that would be irrelevant to people who would accept the “single grammatical unit” notion.
These are the things I started my blog for, but few people go to it so I decided to respond this way here and then drop it.