Yes, the question mark makes it sound like a question. An option is "by Adam Smith". Trying to "
sound more polite" may be the problem - I prefer either brutal directness or genuine kindness to sounding polite.
It's a very well-known quote. IMHO the language and phrasing are too archaic to come across as an opinion, even to someone it isn't well-known to. The curious will google (though it doesn't hurt for you to add attribution).
I hope you'll appreciate that my reply was helpful and charitable - though it too seems to have missed the mark!
