castle in the air, n.All

Castle in the air (n.) in ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠genuine boyhood is a settlement on a desert island, at the happy age when the child knows no other happiness but food and freedom.

Castle in the air (n.) would ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠be strongholds of truth as firm as Euclid's demonstrations, if knowledge were only ideas agreeing — a harpy is not a centaur would be as true as a square is not a circle.