Objection

Here's a snippet from a recent Slate piece on Susan Crawford and the definition of torture:

Her principle objection to detainee abuse is not ephemeral or spiritual....

Oh, argh.

"Principle" is only ever a noun and not ever, ever an adjective, as its place in the sentence would demand: it means "general or fundamental rule". "Principal", on the other hand, can be an adjective meaning "foremost", and this was the intended word. They're both from the same source, Latin "princeps", "[one] that takes first", but they entered the language separately, almost a hundred years apart (both from Old French, "principal" in 1290, "principle" in 1380, each identical then to their current forms), and they do not overlap in meaning. (I was tempted to say "they have never overlapped in meaning", but I can't prove that never once in their centuries-old history have they done so. I doubt it, though.)

Such a mistake is not significantly different from mixing up "cord" and "chord", which likewise come from a common source and which likewise are not interchangeable. Ordinary people might occasionally make such mistakes, but professional writers and their editors are expected to know better.

"Princeps", by the way, is a compound word, from "primus", "first", and "capere", "to take", so a princeps, or a prince, is someone who takes his place as first among citizens.