四明十義書 2.139

2.139

是知不取佛所得圓常自在之法，但用眾生所對色心為所觀之境，故觀所託之境不出三科也。

From this we know that one does not take the perfectly eternal, sovereign dharmas attained by the Buddha, but merely uses the form and mind confronting sentient beings as the object of contemplation; thus the objects upon which contemplation relies do not go beyond the three classifications.