Carving jade along its natural grain — work WITH the stone's inner pattern, not against it. That hidden grain became 'principle, reason', and following it became 'to manage'.
有道理 'that makes sense' is everyday talk; 物理 physics is 'the grain of things', 心理 psychology 'the grain of the mind'. 理发 'managing hair' is just a haircut.
Carve JADE along its grain — find the inner LOGIC of things.
宋 dynasty 理学 'the study of principle' (Neo-Confucianism) dominated Chinese thought for 700 years — all built on this one character.