明道如费, 进道如退, 夷道如类. To categories (Dao) is to restrict it. The entrance and exit are one and the same. Waste not to fully understand (Dao).

上士闻道 , 仅而行之.

中士闻道 , 若存若亡.

下士闻道 , 大笑之.

The fool laughs at it. The smart question its existence. The wise simply follows it.