Ideals shouldn't become pipe dreams - you have to keep your feet on the ground.
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history
The image goes back to 《史记·天官书》, which describes 蜃气 (mirages on the sea) appearing as 楼台 - towers in the air. The phrase 空中楼阁 later crystallized to name any fine-looking but baseless construction.
usage
Pejorative - describes plans, theories, or promises lacking a solid foundation. Common frame: 成为 / 只是 / 不过是空中楼阁. Pairs naturally with 脱离实际, 脚踏实地 (as a corrective).
