People can only see themselves in the mirror (or photos, in short, they cannot see themselves like you see your neighbors). This means that you are always a person in the eyes of others and you may not know it yourself. What you are like. A simple sentence describes the complexity of human nature and the exploration of human nature and the relationship between people and the world. It is very philosophical.
Taking into account the expressions of European humanists and the translation habits of our translators, I think this is the general meaning
It is my personal understanding of this sentence. Just kidding!