The chain is also known as the twelve Nidanas or causes.  It is clearly in its positive and negative forms an amplification of the second and third truths respectively, or perhaps they are a luminous compendium of it.

Besides the full form quoted above there are shorter versions.  Sometimes there are only nine links[444] or there are five links combined in an endless chain[445].  So we must not attach too much importance to the number or order of links.  The chain is not a genealogy but a statement respecting the interdependence of certain stages and aspects of human nature.  And though the importance of cause (hetu) is often emphasized, the causal relation is understood in a wider sense than is usual in our idiom.  If there were no birth, there would be no death, but though birth and death are interdependent we should hardly say that birth is the cause of death.

In whatever way we take the Chain of Causation, it seems to bring a being into existence twice, and this is the view of Buddhaghosa who says that the first two links (ignorance and the sankharas) belong to past time and explain the present existence:  the next eight (consciousness to existence) analyse the present existence:  and the last two (birth and old age) belong to future time, representing the results in another existence of desire felt in this existence.  And that is perhaps what the constructor of the formula meant.  It is clearest if taken backwards.  Suppose, the Buddha once said to Ananda[446], there were no birth, would there then be any old age or death?  Clearly not.  That is the meaning of saying that old age and death depend on birth:  if birth were annihilated, they too would be annihilated.  Similarly birth depends on Bhava which means becoming and does not imply anything self-existent and stationary:  all the world is a continual process of coming into existence and passing away.  It is on the universality of this process that birth (jati) depends.  But on what does the endless becoming itself depend?  We seem here on the threshold of the deepest problems but the answer, though of wide consequences, brings us back to the strictly human and didactic sphere.  Existence depends on Upadana.  This word means literally grasping or clinging to and should be so translated here but it also means fuel and its use is coloured by this meaning, since Buddhist metaphor is fond of describing life as a flame.  Existence cannot continue without the clinging to life, just as fire cannot continue without fuel[447].

The clinging in its turn depends on Tanha, the thirst or craving for existence.  The distinction between tanha and upadana is not always observed, and it is often said tanha is the cause of karma or of sorrow.  But, strictly speaking, upadana is the grasping at life or pleasure:  tanha is the incessant, unsatisfied craving which causes it.  It is compared to the birana, a weed which infests rice fields and sends its roots deep into the ground.  So long as the smallest piece of root is left the weed springs up again and propagates itself with surprising rapidity, though the cultivator thought he had exterminated it.  This metaphor is also used to illustrate how tanha leads to a new birth.  Death is like cutting down the plant:  the root remains and sends up another growth.