大乘起信論義記 2.238

2.238

First, as for "the mark of sameness": the two dharmas of defilement and purity both have tathatā as their substance, and tathatā has these two dharmas as its marks; therefore it says, "sharing the nature and marks." The "various earthenware vessels" illustrate the dharmas of defilement and purity. "All sharing the nature of dust" means: the vessels have dust as their nature, and dust has the vessels as its marks; hence it says "the nature and marks of fine dust."