'insight', is the intuitive light flashing forth and exposing the truth of the impermanency, the suffering and the impersonal and unsubstantial nature of all corporeal and mental phenomena of existence. It is insight-wisdom (vipassanā-paññā) that is the decisive liberating factor in Buddhism, though it has to be developed along with the 2 other trainings in morality and concentration. The culmination of insight practice (s. visuddhi VI) leads directly to the stages of holiness (s. visuddhi VII).
Insight is not the result of a mere intellectual understanding, but is won through direct meditative observation of one's own bodily and mental processes. In the commentaries and the Vis.M., the sequence in developing insight-meditation is given as follows:
The stages of gradually growing insight are described in the 9insight- knowledge (vipassanā-ñāna), constituting the 6th stage of purification: beginning with the 'knowledge of rise and fall' and ending with the 'adaptation to Truth'. For details, see visuddhi VI and Vis.M. XXI.
Eighteen chief kinds of insight-knowledge (or principal insights, mahā-vipassanā) are listed and described in Vis.M. XXII, 113:
(1) contemplation of impermanence (aniccānupassanā),
(2) of suffering (dukkhānupassanā),
(3) of no self (anattānupnupassanā),
(4) of aversion (nibbidānupassanā).
(5) of detachment (virāgānupassanā),
(6) of extinction (nirodhānupassanā),
(7) of abandoning (patinissaggānupassanā),
(8) of waning (khayānupassanā),
(9) of vanishing (vayānupassanā),
(10) of change (viparināmānupassanā),
(11) of the unconditioned (or signless, animittānupassanā),
(12) of desirelessness (apanihitānupassanā),
(13) of emptiness (suññatāupassanā),
(14) insight into phenomena which is higher wisdom (adhipaññā-dhamma-vipassanā),
(15) knowledge and vision according to reality (yathā-bhūta-ñānadassana),
(16) contemplation of misery (or danger, ādīnavānupassanā),
(17) reflecting contemplation (patisankhānupassanā),
(18) contemplation of turning away (vivattanānupassanā).
Through these 18, the adverse ideas and views are overcome, for which reason this way of overcoming is called 'overcoming by the opposite' (tadanga-pahāna, overcoming this factor by that). Thus
(1) dispels the idea of permanence.
(2) the idea of happiness,
(3) the idea of self,
(8) the idea of compactness,
(10) the idea of lastingness,
(11) the conditions,
(14) grasping and adherence to the idea of substance,
(15) attachment and adherence,
(18) dispels entanglement and clinging.
Insight may be either mundane (lokiya) or supermundane (lokuttara). Supermundane insight is of 3 kinds:
(1) joined with one of the 4 supermundane paths,
(2) joined with one of the fruitions of these paths,
(3) regarding the extinction, or rather suspension, of consciousness (s. nirodha-samāpatti).
See samatha-vipassanā, visuddhi, III-VII.