The understanding of conditioned co-arising is so central to Buddhist practice and development that the Buddha’s chief disciple, Sāriputta, said, “Whoever sees Conditioned Co-arising sees Dhamma  , whoever sees Dhamma  sees Conditioned Co-arising” (MN.I.191). Moreover, after his enlightenment, the Buddha is said to have reflected on what he had discovered, initially feeling that it was too subtle for others to understand:

This Dhamma won by me is profound (gambhira), difficult to see, difficult to understand, peaceful, sublime, beyond the scope of mere reasoning (atakkāvacara), subtle, to be experienced by the wise. But this generation is delighting in clinging (to the familiar) . . . so that this were a matter difficult to see, that is to say specific conditionality (idappaccatyatā), Conditioned Co-arising. This too were a matter difficult to see, that is to say the stilling (samatha) of all constructing activities, the renunciation of all attachment, the destruction of craving, dispassion, stopping (nirodha), Nirvāna.

(MN.I.167)

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in the term paticca-samuppāda , samuppāda comes from sam, “together,” and uppāda, “arising.” As explained by the fifth-century Theravādin commentator Buddhaghosa, this means that something can only arise when its conditions are gathered together (Vism.521). Something arises together with its conditions. Paticca means “conditioned,” “having fallen back on,” “grounded on,” being derived from pati-i, from which comes the verb pacceti, “it falls back on.” From the same root comes the word paccaya, “condition” or “foundation.” Synonyms for paccaya are nidāna , “ground,” hetu , “cause,” samudaya, “origin,” āhāra, “nutriment,” and upanisa, “support.” Thus a paccaya is a supporting ground which helps to set off and feed that which it conditions. Paticcasamuppāda thus means something like conditioned co-arising, grounded co-arising, arising together with conditions.
What of the term nirodha , “stopping” or “cessation,” in the conditioned co-arising formula? Does this refer to the stopping of a particular instance of “birth,” for example, or to the stopping of the whole process of births in a person? It is clear that the latter is meant. DN.II.57 talks of “if there were absolutely no birth at all . . . with the cessation of birth, could aging-and-death appear?” As one Therav ā din commentary (MN-a. II.308) puts it, “stopping” is equivalent to “non-arising (an-uppāda)”: it is the stopping of the process of the rise and fall of instances of, say, feeling. Conditioned phenomena are both constantly arising and passing away, but are also subject to final “stopping” or “cessation.” The emphasis is on how types of things arise, so that they can be changed or stopped.

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By becoming aware of how one is conditioned, one can come to alter the flow of conditions by governing, suspending, or, for skilful ones, intensifying them so as to reduce dukkha, and ultimately stop it entirely by transcending the conditions: reconditioning, then de-conditioning.
Nirvāna is the stopping of the entire sequence of conditions mapped out in the conditioned co-arising teaching. With the arising of the Dhamma-seeing Dhamma-eye at stream-entry, which knows “whatever is of the nature to arise (samudaya-dhamma), all that is of the nature to stop (nirodha-dhamma),” there is insight into both the way in which the nidānas arise in the conditioned co-arising sequence, and that these conditionally arisen dhammas are of such a nature that they can be stopped/transcended in the “stopping/cessation” that is nirvāna . The Dhamma-eye thus sees the four True Realities for the Spiritually Ennobled in seeing: conditioned dukkha states, how they arise, how they stop when their conditions stop, and the Noble Eightfold Path (itself the “best of all conditioned states,” AN.II.34) as the way to this. The Stream-enterer knows all the conditions and how they can be stopped, and so “stands squarely before the door of the deathless” (SN.II.43): he or she can “see” the nirvāna that will later be fully experienced at arahatship.

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Besides explaining the origin of dukkha, the formula also explains karma, rebirth, and the functioning of personality, all without the need to invoke a permanent self. No substantial self can be found which underlies the nidānas, owning and operating them: they simply occur according to conditions. Thus it is inappropriate to ask, for example, “who craves?,” but appropriate to ask what craving is conditioned by, the answer being “feeling” (SN.II.14) [emphasis added – see previous post on William James]. Just as Buddhism looks at “how?” rather than “why?” questions, it also looks at “how?” rather than “who?” questions. Nevertheless, in the context of moral discourse, it treats any particular conditioned stream of mental and physical processes as a “person” who is held (except for extenuating circumstances) responsible for “his” or “her” actions. Hence, you are responsible for your actions even though no essential “You” can be found who is their agent.
While the five khandha doctrine is an analysis of the components of personality in static form, the 12 nidāna formula is a synthesis, which shows how such components arise (SN.II.28) and interact dynamically to form the living process of personality, in one life and from life to life. Each of the five khandhas also occurs in the nidāna formula. Consciousness, constructing activities, and feeling occur in both lists. Material form (rupa) is the same as the “body” (part of link 4), and perception (saññā) is part of “mind” (nāma); in the form of misinterpretation, it is also tantamount to spiritual ignorance.

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The nidāna of spiritual ignorance is defined as unknowing (aññāna) with regard to the Four True Realities for the Spiritually Ennobled (SN.II.4). As the principle of conditioned co-arising underlies these truths, the first link can be seen, ironically, to be ignorance of this very principle. Conditioned co-arising, then, is a process which can operate only in ignorance of itself. Once a person fully understands it, it can be stopped. The “ignorance” referred to is not lack of information but a more deep-seated misperception of reality, which can be destroyed only by direct meditative insight.
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Buddhism, then, sees the basic root of the pain and stress of life as spiritual ignorance, rather than sin, which is a willful turning away from a creator God. Indeed, it can be regarded as having a doctrine of something like “original sinlessness.” While the mind is seen as containing many unskillful tendencies with deep roots, “below” these roots it is radiant: “Monks, this mind (citta) is brightly shining (pabhassara ; Skt. prabhāsvara), but it is defiled by adventitious defilements” (AN.I.10). That is, the deepest layer of the mind is bright and pure (though not yet immune from being obscured by defilements). This represents, in effect, the potentiality for attaining nirvāna – but defilements arise through inept interaction of the mind with the world. The idea of defilements as “arriving” or “adventitious” is related to their non-Self nature: they are not an intrinsic part of person, so can be transcended. Even a newborn child is not seen as having a wholly pure mind, however, for it is said to have unskillful latent tendencies  (anusaya) […] In the calm of deep meditation, the depth-radiance of the mind is experienced at a conscious level as the process of meditation suspends the defiling five hindrances, just as a smelter purifies gold ore so as to attain pure gold (SN.V.92). More than a temporary undefiled state of mind is necessary for awakening, however. For this, there must be destruction of the four “taints” or “cankers” (āsava; Skt. āsrava ): the most deeply rooted spiritual faults, which are likened to festering sores, leeching off energy from the mind, or intoxicating influxes on the mind. These are the taints that flow in relation to sense-desire, becoming, views, and spiritual ignorance, which are seen as conditioning, and being conditioned by, spiritual ignorance (MN.I.54–5). One can see ignorance, indeed ignore-ance, as a misperception which beclouds the basic radiance of mind. One can perhaps see craving as leading to the willful ignoring of things that one has, in part of one ’ s mind, or at some past time, realized. People are good at forgetting. This is one reason why Buddhism emphasizes mindfulness, which includes an element of careful “bearing in mind.”

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Ignorance can be seen to condition active impulses in that all actions are performed from the perspective of a particular way of perceiving and construing the world, an outlook and set of beliefs, which provides a motivating framework: a person acts in response to the “world” as it appears to him or her. Prior to enlightenment, all actions will be in some way affected by misperceptions, or at least by correct beliefs which are not based on direct perception, so as to be in some way narrow or incomplete. Actions can bring positive fruits if they are based on some degree of insight into reality, such as the principles of karma or impermanence. In a person who has destroyed spiritual ignorance, though, actions no longer have the power to “construct” any karmic results.

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A person, then, consists of a dynamic interplay between consciousness and the body of other mental and physical states that are either the objects of consciousness or its facilitating complements. In the vortical interplay between consciousness and nāmarupa (Harvey 1995 , 116–21), the whole complex of the 12 links of conditioned coarising and the realm of language is spun out:

just this, namely nāma-rupa , is the cause, ground, origin and condition of consciousness. Thus far, then, can we trace birth and decay, death and passing away and being reborn, thus far extends the way of designation (adhivacana-), of language (nirutti-), of concepts (paññatti-), thus far is the sphere of understanding (paññāvacara), thus far the round (of rebirth) goes as far as can be discerned here, namely nāma-rupa together with consciousness.

(DN.II.63–4)

Consciousness hangs around nāma-rupa , the other four khandhas , as its “home” (SN.III.9–10). True renunciation, non-attachment to this conditioned “home,” opens up the possibility of radical “homelessness”: the realm of the unconditioned, nirvāna. When there is no craving or grasping, consciousness can be like a sunbeam that lands nowhere, being “unsupported (appatitthita)” (SN.II.101–5). This simile suggests that consciousness that has “stopped,” being no longer meshed in the network of conditions, does not stop existing, any more than a radiant sunbeam does when it is not obstructed by anything.