CATHOLIC CHURCH: No meat on Fridays
PEOPLE: Ok so no eggs for breakfast
CC: No, eggs are ok
P: But eggs are chicken
CC: Not until they hatch
P: So the thing isn't the thing until it's born?
CC: Correct—um, no, wait.
@juglugs All kidding aside, why no meat? And why does seafood not count as Meat?

@jpwkeeper

Economics. There simply wasn't enough meat to go around. So the churches said it was made-up beings telling people that they couldn't eat on Fridays.
Plenty of fish in the sea (then).
And these were uneducated peasants, so they didn't realise that fish were animals let alone meat...