四明十義書 2.90

2.90

若眾生諸佛為外境，則觀眾生陰入色心成真淨色心也。諸佛雖離陰入，行人所觀須將應身色心為境。

If sentient beings and all buddhas serve as external objects, then one contemplates the form and mind of the aggregates and sense bases of sentient beings to accomplish truly pure form and mind. Although all buddhas are free from the aggregates and sense bases, the practitioner's contemplation must take the form and mind of the response body as the object.