四明十義書 1.109

1.109

須知以性融攝故，則內境外境有相趣之義。約事分內外故，則觀內不可放心緣外，觀外不可放心緣內。

One must know that because of fusion and subsumption by the nature, internal objects and external objects have the meaning of mutually converging. Because internal and external are distinguished in terms of phenomena, when contemplating the internal, one must not let the mind wander to conditions outside; when contemplating the external, one must not let the mind wander to conditions inside.