I am looking for a proverb (or a famous saying) which states that if you travel to only one country, you will not be able to understand all its features because you haven't seen the other countries, and you cannot make a comparison between them (Actually I have heard that People who live in London say such a thing about their own country, claiming that London is the most beautiful country in the world.)
Or, if you have only read the Bible, for example, you will not understand all the nuanced points of it because you need to read other holy books as well.
Or, in other cases, suppose you want to determine if a specific grammar book is good or not.
How can you decide? By examining other grammar books as well.
Do we have a related proverb in English?