The phrase "what is essential is invisible to the eye" is a quote from Antoine de Saint-Exupérys book, The Little Prince. In the book, the little prince explains to the castaway that real seeing is not even a physical activity but a matter of the heart. The full quote is "It is only with the heart that one can see rightly; what is essential is invisible to the eye". The quote suggests that what is truly important in life cannot be seen with the eyes, but rather felt with the heart. It emphasizes the importance of looking beyond appearances and material possessions to see the true essence of things and people.