涅槃經遊意 1.221

1.221

次有新安瑤師云：眾生有成佛之道理，此理是常，故說此眾生為正因佛性。此理附於眾生，故說為本有也。

Next, Master Yao of Xin'an says: sentient beings possess the principle of attaining Buddhahood; this principle is eternal, and therefore these sentient beings are said to be the Buddha-nature as primary cause. Because this principle attaches to sentient beings, it is said to be originally existent.