Plotinus (204-270 CE) ends the Enneads with the ascent to the One: the soul passes purification, Intellect, and finally a union (henosis) that is not knowledge, since knowledge implies two, but presence, 'the flight of the alone to the Alone' (Ennead VI.9.11). Porphyry's Life of Plotinus reports Plotinus achieved this union four times in five years, and his dying words, 'try to raise the god in you to the divine in the universe', close the dossier.
Modern mysticism catalogs quote Plotinus as proof that union experiences are universal, ignoring his warning that the One is beyond every experience, since experiences have subjects. The Enneads read less like a trip report than like a rigorous account of why trip reports fail.
Plotinus's system is a metaphysics that predicts its own failure: emanation explains everything except how to return, and the final step, erasing the distinction between seer and seen, cannot be described by the system that motivates it. Philosophy, for Plotinus, is the scaffolding you kick away.
If union is beyond thought, what are the five volumes of philosophy for?