大乘起信論廣釋卷第三．四．五 3.48

3.48

[Treatise:] In this way, undefiled [and defiled dharmas … down to …] nature and characteristic of tathatā.
[Commentary:] This is the application to dharma. Initial and original awakenings are named 'undefiled'; fundamental and derivative non-awakening are named 'avidyā'. Because they have functioning that manifests, they are named 'karma'; yet because they are not truly existent, they are named 'illusion'. Although defilement and purity differ, both have tathatā as their substance, because the stirring of tathatā constitutes these two. The 'characteristic' here is also substance and nature; it does not mean that they are called nature and characteristic in relation to one another. The new translation merely says 'because they are the characteristic of tathatā'.