We might compare the entrance of man into another world with the passing of a fish from the water into the air; the fish has first to be prepared for this by changing its air bladders into lungs. This resembles the transition from sense perception to spiritual perception, whereby a man’s soul is made capable of employing certain forces in a different element. Many things are then revealed to him.
[…] Man must not remain in the water. If he did, his later life would be “airless” as regards spiritual ideas — he would gasp for air.
Source: Rudolf Steiner – GA 124 – Excursus on the Gospel According to St. Mark: Lecture VII – Berlin, 13th March 1911