The Buddha said there are twelve links (nidanas) in the "chain" of Dependent Origination. Dependent Origination is the Buddhist teaching on how things come to be, are, and cease to be. According to this teaching, no beings or phenomena exist independently of other beings and phenomena.
The Twelve Links show us how Dependent Origination "works." Please note that there is no first link. The "numbering" could begin anywhere. Although they are described in a linear way, in fact each link connects to all the other links.
1. Ignorance (Avidya)
Ignorance also refers to ignorance of anatman, a teaching that there is no "self" in the sense of a permanent, integral, autonomous being within an individual existence. What we think of as our self, our personality and ego, are temporary creations of the skandhas.The twelve links are illustrated in the outer ring of the Bhavachakra (Wheel of Life). Ignorance is depicted as a blind man or woman.
Ignorance conditions the next link in the chain, volitional action.
2. Volitional Formation (Samskara)
Ignorance links to samskara, which can be translated volitional action, formation, impulse, motivation, or karma formation. Out of ignorance come thoughts, words and actions that sew the seeds of karma. Samskara are the impulses that come before action, and action creates karma.
In the outer ring of the Bhavachakra (Wheel of Life), samskara usually is illustrated as potters making pots.
Volitional formation conditions the next link, consciousness.
3. Consciousness (Vijnana)
Vijnana usually is translated to mean "consciousness." In Buddhist teaching there are many kinds of vijnana. Very generally, vijnana is what happens when one of the six faculties (eye, ear, nose, tongue, body, mind) reacts to or becomes aware of one of the six external phenomena (visible form, sound, smell, taste, touch, and ideas and thoughts). The third link includes all kinds of vijnana.
In the outer ring of the Bhavachakra (Wheel of Life), vijnana is represented by a monkey. A monkey leaps thoughtlessly from one thing to another, easily tempted and distracted by sensations. Monkey energy pulls us away from ourselves and away from the dharma.
Vijnana conditions the next link, name and form.
4. Name and Form (Nama-rupa)
Nama-rupa is name and form, corporeality and mentality, the joining of the five skandhas into an individual existence. With name and form also come sensory perception.
Nama-rupa works together with the next link, faculties and objects, to condition other links.
5. Faculties and Objects (Shadayatana)
The Vijnana, or consciousness, link described above involves the the six faculties or sense organs (eye, ear, nose, tongue, body, mind) and six corresponding external phenomena, or objects (visible form, sound, smell, taste, touch, and ideas and thoughts). The faculties and their corresponding objects are the shadayatana.
Of course, the sense organs do not exist separately from the previous link, physical form. If you reflect on the links, you see that each link connects to every other link.
The Bhavachakra (Wheel of Life) illustrates shadayatana as a house with six windows.
Shadayatana relates directly to the next link, which is the contact between faculties and objects.
6. Contact (Sparsha)
Sparsha is contact with environment, or the contact with the faculties and object discussed of Shadayatana. The Wheel of Life illustrates sparsha with an embracing couple.
The contact of faculties and objects leads to the experience of sensation, which is the next link.
7. Sensation (Vedana)
Vedana is the recognition and experience of sensations. These experiences are pleasurable or painful, which leads to desire and aversion.
The Wheel of Life illustrates vedana with an arrow piercing an eye to represent sense data piercing the senses.
Sensation conditions the next link, craving.
8. Craving (Trishna)
If we are not mindful, we are perpetually being jerked around by desire for what we want and aversion of what we don't want. In this state we heedlessly create karma, which keeps us entangled in the cycle of rebirth.The Wheel of Life illustrates trishna with a man drinking beer, usually surrounded with empty bottles.
Desire and aversion conditions the next link, clinging.
9. Clinging or Grasping (Upadana)
Upadana is a grasping and clinging mind. We cling to sensual pleasures, mistaken views, external forms and appearances. Most of all, we cling to ego and a sense of an individual self, a sense reinforced moment-to-moment by our cravings and aversions. Upadana also represents clinging to a womb and the beginning of rebirth.
The Wheel of Life illustrates Upadana by picturing a monkey, or sometimes a person, reaching for a fruit.
Upadana conditions the next link, becoming.
10. Becoming (Bhava)
Bhava is new becoming, set in motion by the other links. The Wheel of Life illustrates bhava by picturing a couple making love or a woman in an advanced state of pregnancy.
Becoming conditions the next link, birth.
11. Birth (Jati)
The cycle of rebirth naturally includes birth. In the Wheel of Life, a woman in childbirth illustrates jati.
Birth inevitably leads to old age and death.
12. Old Age and Death (Jara-maranam)
The chain comes to old age and death, or the dissolution of what came to be. The karma of one life sets in motion another life, rooted in ignorance (avidya). A circle is closed.
In the Wheel of Life, Jara-maranam is illustrated with a corpse.
The Four Noble Truths teach us that release from the cycle of samsara is possible. In the resolution of ignorance, volitional formations, craving and grasping there is liberation from birth and death and the peace of nirvana.