castle in the air

“Before you started building castles in the air,” Josie said loyally.

All entrepreneurs making pitches to venture capital funds are inclined to promise castles in the air and riches beyond the dreams of Croesus, or they won’t be invited through the door.

“Wouldn’t it be fun if all the castles in the air which we make could come true, and we could live in them?” said Jo, after a little pause.

"At a time when the pandemic is killing thousands, crematoria are full and graveyards have run out of space, the government is building castles in the air."

In the Scientific Revolution, Bacon and Descartes were amongst those with plans for thoroughgoing intellectual change, but their plans were castles in the air, and neither of them imagined what Newton would achieve.