Based on your experience, what do you think is that that is essential but invisible to the eye?

Air

Companionship and friendship - both essential for surviving daily life. Neither one is visible to the eye.

A support network - it doesn't matter what kind of trial a person is going through, if they have a support system, they will make it. A support network cannot be seen, but it can be felt in the heart and known in the spirit.

Love - love is an essential emotion. Whether it is love of another individual, the love of a pet, the love of a family, the love of a hobby, it matters not. But to me, it is essential. Again, it cannot be seen.

Those are just a few of my thoughts.