People can only see themselves in the mirror (or photos, in short, you can't see yourself like your neighbors), which means that you will always be alone in the eyes of others, and you may not even know what you look like. In a simple sentence, it describes the complexity of human nature and discusses the relationship between man and the world, which is very philosophical. Considering the expression of European humanists and our translation habits, I think this is probably what it means.

It is my personal understanding of this sentence.