This is not a duplicate of “anymore” vs. “any more”. I made sure. In that case, the two terms being compared were noun phrase (any more) vs. adverb (anymore). In this case, the two are both noun terms. I know which is more often used, but is “any thing” incorrect?
The question was raised in my mind when I saw “any thing” on a post, and wondered whether to edit it to “anything”.
I think I would find myself intuitively using “any thing” on those rare occasions that “thing” needed to be a ‘noun in its own right’, and contrast with other nouns. For example:
“Any thing or person seen in the garden….”
“*Anything or person seen…”
I didn’t see any ‘thing’ as such, but rather a group of things.
Other than cases such as these, I don’t think there’s much in it. In many cases where you could argue that what is meant is strictly a ‘thing’ (and where you could replace “any( )thing” with “a single thing”), replacing “any thing” with “anything” doesn’t really alter the meaning:
I don’t believe anything/any thing/a single thing you say.
probably since in cases such as this, ‘thing’ doesn’t contrast with another noun.