大乘起信論義記 3.271

3.271

In the second [section], as for the phrase "even that [...] nirvāṇa, tathatā, and so on are ultimately empty and free from all characteristics": the Pañcaviṃśatisāhasrikā Prajñāpāramitā Sūtra says, "Even nirvāṇa is like an illusion and like a dream. If there were any phenomenon surpassing nirvāṇa, I would say that it, too, is like an illusion and like a dream." Because one does not know that this was meant to demolish the conceptual positing of existence by the deluded mind, one grasps the virtues of nature as being solely non-existent; hence [the text] says, "Because one does not know..."