大乘起信論廣釋卷第三．四．五 3.46

3.46

[Treatise:] What are the two? [… down to …] and second is the characteristic of difference.
[Commentary:] These are the specific names. Defilement and purity being equal in tathatā is called 'identity'; the meaning of identity can be expressed, so it is named 'characteristic'. Defilement and purity being distinct in substance is called 'difference'; having distinct forms is called 'characteristic'. Why are there identity and difference like this? Because substance conforms to conditions, there is difference; because causal conditions are subsumed back into substance, there is identity. Because conditions conform to substance in identity, tathatā has a single flavor; because substance conforms to conditions in difference, ordinary beings and noble ones are divided into two. Because ordinary beings and noble ones are divided, the meaning of conventional truth is established; because tathatā is one, the principle of ultimate truth remains. Identity and difference of awakening and non-awakening are like this.