Nagarjuna
Two truths

Nāgārjuna was also instrumental in the development of the two truths doctrine, which claims that there are two levels of truth in Buddhist teaching, the ultimate truth (paramārtha satya) and the conventional or superficial truth (saṃvṛtisatya). The ultimate truth to Nāgārjuna is the truth that everything is empty of essence, this includes emptiness itself ('the emptiness of emptiness'). While some (Murti, 1955) have interpreted this by positing Nāgārjuna as a neo-Kantian and thus making ultimate truth a metaphysical noumenon or an "ineffable ultimate that transcends the capacities of discursive reason", others such as Mark Siderits and Jay L. Garfield have argued that Nāgārjuna's view is that "the ultimate truth is that there is no ultimate truth" (Siderits) and that Nāgārjuna is a "semantic anti-dualist" who posits that there are only conventional truths. Hence according to Garfield:

Suppose that we take a conventional entity, such as a table. We analyze it to demonstrate its emptiness, finding that there is no table apart from its parts. So we conclude that it is empty. But now let us analyze that emptiness. What do we find? Nothing at all but the table's lack of inherent existence.. To see the table as empty is to see the table as conventional, as dependent.

In articulating this notion in the Mūlamadhyamakakārikā, Nāgārjuna drew on an early source in the Kaccānagotta Sutta, which distinguishes definitive meaning (nītārtha) from interpretable meaning (neyārtha):

By and large, Kaccayana, this world is supported by a polarity, that of existence and non-existence. But when one reads the origination of the world as it actually is with right discernment, "non-existence" with reference to the world does not occur to one. When one reads the cessation of the world as it actually is with right discernment, "existence" with reference to the world does not occur to one.

By and large, Kaccayana, this world is in bondage to attachments, clingings (sustenances), and biases. But one such as this does not get involved with or cling to these attachments, clingings, fixations of awareness, biases, or obsessions; nor is he resolved on "my self". He has no uncertainty or doubt that just stress, when arising, is arising; stress, when passing away, is passing away. In this, his knowledge is independent of others. It's to this extent, Kaccayana, that there is right view.

"Everything exists": That is one extreme. "Everything doesn't exist": That is a second extreme. Avoiding these two extremes, the Tathagata teaches the Dhamma via the middle...

The version linked to is the one found in the nikayas, and is slightly different from the one found in the Samyuktagama. Both contain the concept of teaching via the middle between the extremes of existence and non-existence. Nagarjuna does not make reference to "everything" when he quotes the agamic text in his Mūlamadhyamakakārikā.
combination:repeat_prompt, detectable_content:postscript