大般涅槃經義記 8.215

8.215

第三段中曲有四句：一明佛性非陰界入；二明佛性非本無今有、已有還無；三明佛性可從緣見；四明佛性非內非外。於中先別鐵色為喻。「鐵」喻眾生。「黑色」喻性。「火」喻煩惱。「赤」喻佛性隨緣成染。合之可知。下別草木種子為喻，略無合文。

In the third section, there are in detail four phrases: 1. clarifying that buddha-nature is not the aggregates, realms, or sense-spheres; 2. clarifying that buddha-nature is not originally non-existent and now existent, nor existing before and returning to non-existence; 3. clarifying that buddha-nature can be seen through conditions; 4. clarifying that buddha-nature is neither internal nor external. Therein, it first separately uses the color of iron as an analogy. "Iron" is an analogy for sentient beings; the "black color" is an analogy for the nature; "fire" is an analogy for defilements; "red" is an analogy for buddha-nature conforming to conditions and becoming defiled. The correlation is evident. Below, it separately uses seeds of grass and trees as an analogy, the text of the correlation being omitted.