The teaching on the twelve links of dependent origination is one of the Buddha’s greatest insights. It is essentially a model of reality, causality, perception, and awakening all in one. You can find it in its original form in such places as sutta 15 of The Long Discourses of the Buddha, DN 15, ”The Great Discourse on Causation/Origination”, also called “The Great Causes Discourse”. Fragments of it appear in numerous other suttas. There are many books and essays that go into the depths of this subject, so if you want something at that level, look there, not here. In an ultra-simplified nutshell, it lays out the chain of causality in which each thing is caused by something else until we get to the root cause. The premise is that if the root cause is removed, then the great chain of causality collapses. This great chain of causality was contemplated by the Buddha shortly after his awakening and represents one of his most profound teachings.
Starting at the top, we have the obvious and undeniable fact that sickness, aging, pain, lamentation, grief, despair, and death are dependent on birth. That link in the chain is an important thing to notice and get to know very well, as, curiously enough, studying that link well leads to understanding the whole thing. Beginning the moment you are born (conceived, actually), you will age, have pain, get sick, and die. Makes sense. The number of people who are in denial of this basic and obvious set of facts is mind-boggling. Don’t be one of those.
Then this gets stranger: “birth” is based on “becoming” (also translated as “existence”). Here we must suddenly dive into the world of ancient (and often modern) India, with its working paradigm that envisioned beings are reborn into countless lives again and again based on their karma. I am going to skip over the complexities (such as what it is that transmigrates), as it is not terribly important to the main points I wish to make here. The teaching on the twelve links makes great sense once you get sufficiently awakened and doesn’t necessarily require belief in the doctrine of rebirth. (All you scientific materialists and nihilists can relax any reflexively constricted orificia.)
Speaking of constriction, “becoming” is based on “clinging”, in that we cling to existence. This clinging is based on “craving” for experience and for being. This craving is based on vedana (usually translated as “feeling” or perhaps “feeling tone”), which is the aspect of sensations being pleasant, unpleasant, and neutral. Vedana arises in dependence on “contact” (between objects and the sense doors), and contact depends on “the six sense doors”. These six sense doors are themselves dependent on “name and form” (which refer to mind and matter). Name and form are themselves dependent on “consciousness”. Consciousness is dependent on “volitional formations” (san˙khāras in Pali, a word that has a range of meanings and is worth looking up and reading more about). And, last but not least, volitional formations are based on “ignorance”.
Each of those terms is complicated, and their very specialized meanings and implications are also complicated. The translation makes these difficulties worse. Potentially having a totally different working paradigm from those prevalent in ancient and modern India regarding rebirth may not help at all. However, a few salient points can be made.
First, the elimination of ignorance entails direct realization of the three characteristics, meaning a highly developed degree of awakening to the natural and basic truth of all sensate phenomena, everything that is actually experienced. In this way of perceiving, reality is no longer broken up at a core perceptual level into perceiving subject and perceived object. Nor is anything taken to be unchanging, static, or continuous at a very basic perceptual level. Thus, nothing comprises, forms, or fabricates a deluded sense of an autonomous self, a volitional self, a doer, controller, a “this”, an agent, a separate, permanent, split off, centrally perceiving, independently existing consciousness, subject, awareness, or watcher. This becomes hardwired at the sensate level through clear perception and investigation of the sense doors. At this point in practice, sensate reality is no longer split up artificially into name and form (mentality and materiality, thought and the other five sense doors), as each is just part of the variety of qualities occurring in the fluxing volume of experience.
This then has profound implications for the experience of the six sense doors, as now there is, for lack of a better way to put it, only one wide-open, volumetric sense door and it is sensing itself. Thus, there is really no contact, as there is no sense of anything split off that would be contacting some thing “out there”; there are instead just the qualities and textures of transient space. As there are just the qualities and textures of transient space, it can no longer be said that the qualities and textures called vedana belong to anyone, not that vedana isn’t still causal, since it is. At this point in practice, gone is some sense in that undifferentiated field of a split off “this side” that could try to move closer to pleasant sensations (fundamental attraction), move farther away from unpleasant sensations (fundamental aversion), or tune out boring sensations in a way that creates a sense of perceptual duality (fundamental ignorance), this being the basic implication of craving.
Thus, there is also no solidification of anything, as the whole transient field directly and immediately knows its own utter transience, and so the sort of clinging to any sense of a permanent, continuous “self” referred to here is rendered perceptually impossible. Without any possibility of habitual solidification, the special type of becoming or existence referred to here can’t happen, as it is directly known that no separate thing transmigrates, nothing remains, nothing makes up some stable core of perpetuating consciousness or self. That is all so far so good, as it goes. In fact, if you do insight practices well enough, you too will come to see that all that abstract-sounding theory suddenly describes experience to a tee! How cool is that? In fact, it is very, very cool.
However, finally, in the last two links, we have a problem, that being the annoying fine print regarding the end of suffering promised by the Buddha. As clearly demonstrated in the suttas, and as should be obvious, a body was born, and while the body lives, there will be pain, aging, sickness, and finally death. Then, we have the endless debates between the Mahayana and the Theravada about what happens next, but vastly more important to the pragmatist is understanding the links from one end to the other of this lifetime to the degree that they can be understood while a body still lives.