He is broad-minded toward others, and so becomes great - his friends span the world.
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history
From Li Si's (李斯, Qin) 《谏逐客书》: 泰山不让土壤，故能成其大；河海不择细流，故能就其深. (Mount Tai refuses no handful of earth, so it achieves its greatness; rivers and seas turn away no small stream, so they reach their depth.) Li Si's memorial persuaded the future First Emperor to rescind his expulsion of foreign-born officials.
usage
Nearly always quoted after its partner clause, most often 泰山不让土壤 or 海纳百川. The 其 is an anaphoric 'its' pointing back to the subject (mountain/sea/person). Common in speeches on openness, inclusion, and leadership.
