In fact, the realization of the absence of a separate self is widely considered, in Gelugpa, Nyingma and other Tibetan Schools, the very first step in a series of radical shifts.

It is the same first step in mystical Christianity and Judaism, Sufism, Taoism, and many other contemplative traditions.

Take the Bhagavad Gita, for example. It is not entirely linear but it does describe something that can legitimately be referred to as stages. Arjuna learns in Chapter 2 (the 2nd of 18 chapters), about the nature of egolessness.

By Chapter 6 — the chapter on meditation, by the way — he has gone far beyond that, to the stage of Unity, the unity of form and emptiness of which so much is made in the Mahayana schools of Buddhism, the “mystic marriage” of the Western mystical traditions, the union of Siva and Shakti of the Tantras.

Ultimately, to use the author’s words, there is a space between emptiness and the concept of space — and then one has just begun — except there is no ‘one” there (nor any “there” there) any more (and not really any more because time ends as well; well, not really “ends” but, well, just negate every word apophatically and that might be a good beginning!)