To know one aspect of a matter but not the other; to have an incomplete understanding of a situation; to know part of the truth but not the whole truth.
他只看到问题的表面，知其一，不知其二，所以提出的解决方案并不全面。
He only saw the surface of the problem, knowing one aspect but not the other, so the solution he proposed was not comprehensive.
Next Idiom