This is from a rather rare and lengthy book (all of one used copy is available on Amazon for $49), An Introduction to Indian Philosophy (Eighth Edition), by Satischandra Chatterjee and Dhirendramohan Datta (University of Calcutta, 1984). It's one and a half pages out of 428, or three tenths of a percent of the total text, from a book that is such an obscure intellectual curiosity, and this is for educational purposes, so I'm calling this fair use. I tried to fix the typographical errors wherever possible. Pages 146-7:
The positive side of the Madhyamika doctrine; there is reality behind phenomena: it is unconditional and free from change.
The conditionality of things which makes their own nature (svabhava) unascertainable, either as real or unreal, etc., may be regarded as a kind of relativity. Every character of a thing is conditioned by something else and therefore its existence is relative to that condition. Sunya-vada can therefore also be interpreted as a theory of relativity which declares that no thing, no phenomenon experienced, has a fixed, absolute, independent character of its own (svabhava) and, therefore, no description of any phenomenon can be said to be unconditionally true.
To this philosophy of phenomena (or things as the appear to us), the Madhyamikas add a philosophy of noumenon (or reality in itself). Buddha's teachings regarding dependent origination, impermanence, etc., apply, they hold, only to the phenomenal world, to things commonly observed by us in ordinary experience. But when nirvana is attained and the conditions of sense-experience and the appearance of phenomena are controlled, what would be the nature of the resultant experience? To this we cannot apply the conditional characters true of phenomena. The Madhyamikas, therefore, hold that there is a transcendental reality (noumenon) behind the phenomenal one and it is free from change, conditionality and all other phenomenal characters. As Nagarjuna says: "There are two truths, on which Buddha's teaching of Dharma depends, one is empirical (sath-brti-satya) and meant for the ordinary people, another is the transcendental or the absolutely true one (paramartha-satya). Those who do not know the distinction between these two kinds of truths, cannot understand the profound mystery of Buddha's teachings."*
The truth of the order is only a stepping-stone to the attainment of the higher. The nature of nirvana-experience which takes one beyond ordinary experience cannot be described, it can only be suggested negatively with the help of words which describe our common experience. Nagarjuna, therefore, describes nirvana with a series of negatives, thus: "That which is not known (ordinarily), not acquired anew, not destroyed, not eternal, not suppressed, not generated is called nirvana."** As with nirvana so also with the Tathagata or one who has realized nirvana. His nature also cannot be described. That is why, when Buddha was asked what becomes of the Tathagata after nirvana is attained, he declined to discuss the question.
In the same light the silence of Buddha regarding all metaphysical questions about non-empirical things can be interpreted to mean that he believed in a transcendental experience and reality, the truths about which cannot be described in terms of common experience. Buddha's frequent statements that he had realized some profound truth which reasoning cannot grasp, can be cited also to support this Madhyamika contention about the transcendental.***
It may be noted there that in its conception of twofold truth, its denial of the phenomenal world, its negative descriptions of the transcendental, and its conception of nirvana as the attainment of unity with the transcendental self, the Madhyamika approaches very close to Advaita Vedanta as taught in some Upanisads and elaborated later by Gaudapada and Sankaracarya.
* Madhyamika-sastra, chapter 24, karikas 8-9.
** ibid. Chap. 25, karika 3.
*** Vide Prof. Radhakrishnan's article, "The teachings of Buddha by speech and silence," Hibberi Journal, April 1934 for a fuller discussion.