The Seven Nails of Acarya Sri Simha (the Lion of Dzogchen)
(i.e.: Seven types of Union to realise to get the true nature of the Ground; which can be grouped in three fundamental types of ‘Union’:
[U2T] Union of the Two Truths: Union conventionally dependently co-arisen relatively functional impermanent appearances (1st truth - not complete non-existence) <==> and emptiness of inherent existence (2nd truth / not real existence) of all dharmas. One aspect / truth implies / proves the other (<==>). These two truths are inseparable, interdependent, co-defined, co-relative, co-dependent, co-emergent, co-evolving, co-ceasing / co-transcended, in harmony, equal / non-dual / one -- in the non-dual sense of those terms; ex. the two truths are not different / separate / two / dual, not identical / unite / one / non-dual, not both together, not neither. Meaning: Their ‘Union’ is inconceivable, beyond all conceptual proliferations, beyond all extremes & middle. Those two truths are themselves empty of inherent existence <==> because co-dependent, and vice versa. Ex. Union relative <==> absolute; Union appearance <==> emptiness; Union functionality / clarity / luminosity / cognizance <==> and emptiness.
[U3S] Union of the three spheres -- subject, relation / action, object --; or Union of the Two Truths about the three spheres. Ex. Union subject / actor / perceiver / knower / acquirer <==> relation / action / perception / cognition / acquisition <==> object / result / perceived / known / acquired; Union cause / producer <==> causality / production <==> effect / product; Union characteristic <==> characterising <==> characterised; Union owner <==> ownership <==> possession.
[Uopp] Union of opposites; or Union of the Two Truths about the opposites of any duality, triad, quad, etc. Ex. Good vs. bad, pure vs. impure, self vs. other, samsara vs. nirvana, existence vs. non-existence, stillness vs. movement, etc.)
21 states that make up the State of a Buddha
From: Paths and Grounds of Guhyasamaja Tantra
Question: What are the Yogas (States of Union) that constitute Buddhahood?
Arya Nagarjuna: The 21 states of union which are included in the two types of union,
"Union with a factor of abandonment" and "Union with a factor of insight" are:
The state of union of Cyclic Existence (Samsara) and Cessation (Nirvana).
The state of union of the afflicted class of phenomena and the liberated class of phenomena.
The state of union of perceptions of objects with and without aspects.
The state of union of subjective perception and objective phenomena.
The state of union devoid of Eternalism and Nihilism.
The state of union of Emptiness and Compassion.
The state of union of Method and Wisdom.
The union of those with and those without remainder.
The union of the two Selflessnesses.
The union of an Illusory Body and a Clear Light.
The union of Thorough and Gradual Dissolutions.
The union of the Two Truths.
The union of entering and emerging from a Meditative Equipoise (Shamata).
The union of sleep and wakefulness.
The union of Meditative Equipoise and the post meditational period.
The union of mindfullness and forgetfullness.
The union of Bliss and Emptiness.
The union of Objects and Action.
The union of the Generation stage and Completion stage.
The union of Purity and Impurity.
The union of those with and without forms.