“They were good filters of each others’ crap, well except for some of the stuff on “The White Album”.”
By that point a songwriting credit of “Lennon & McCartney” was really more like “Lennon OR McCartney”. And you could usually figure out which one it was according to who sang lead.
The classic story was when Paul sang the first line of “It’s Getting Better All the Time”, immediately John threw in the line, “It can’t get no worse.”