Defenders of cross-cultural mystical experience are right to note that in many widely varying cultures, respected sages have referred to the experience of an ultimate nonduality: a perception that everything, including oneself, is ultimately one. But one might also then rightly ask: which ultimate nonduality?

Nondualism may be the world’s most widespread philosophy, but it can mean different things – not merely different things in different places, but different things in the same place. Members of the Indian Vedānta tradition frequently proclaimed that everything is “one, without a second”, in the words of the Upaniṣads they followed. But they disagreed as to what that meant. Śaṅkara founded the Advaita Vedānta tradition – a-dvaita literally meaning non-dual – which argued that only the one, ultimate truth (sat, braḥman) was real, and all multiplicity and plurality was an illusion. His opponent Rāmānuja agreed that everything is “one, without a second” – but in his Viśiṣṭādvaita (qualified nondual) school, that meant something quite different. All the many things and people we see around us – what Chinese metaphysicians called the “ten thousand things” – are parts of that ultimate one, and they are real, not illusory.