The term 四海 (sì hǎi) is deeply rooted in Chinese cosmology and philosophy. In ancient times, it was a geographical concept defining the known world from the perspective of the “Middle Kingdom” (中国, Zhōngguó). The world was the land within these four seas. However, its more profound meaning comes from Confucianism. The famous saying from the Analects, “四海之内皆兄弟” (sì hǎi zhī nèi jiē xiōng dì), translates to “within the four seas, all are brothers.” This transformed `四海` from a geographical boundary into a moral one. It expresses a powerful ideal of universalism and hospitality, suggesting that moral character and shared humanity are more important than geographical or blood ties. To compare, a Western concept like “the four corners of the Earth” is primarily geographical, describing the vastness of the world in a physical sense. 四海 (sì hǎi), especially in its philosophical usage, carries an additional, powerful layer of social and ethical meaning. It's not just about the size of the world; it's about the potential for every person within it to be treated as family. This reflects the Chinese cultural value of harmony and the importance of human relationships on a grand, idealized scale.
While not used for everyday conversation about “the world” (for which 世界 (shìjiè) is used), 四海 (sì hǎi) is very much alive in more formal, literary, and idiomatic contexts.