Tanhā: lit. 'thirst': 'craving', is the chief root of suffering, and of the ever-continuing cycle of rebirths.;What, o Bhikkhus, is the origin of suffering? It is that craving which gives rise to ever-fresh rebirth and, bound up with pleasure and lust, now here, now there, finds ever fresh delight. It is the sensual craving kāma-tanhā the craving for existence bhava-tanhā the craving for non-existence vibhava-tanhā D. 22. T. is the 8th link in the formula of the dependent origination.
Corresponding to the 6 sense-objects, there are 6 kinds of craving craving for visible objects, for sounds, odours, tastes, bodily contacts, mental contacts rūpa-, sadda-, gandha-, rasa-, photthabba-, dhamma-tanhā M. 9; D. 15
Corresponding to the 3-fold existence, there are 3 kinds: craving for sensual existence kāma-tanhā for fine-material existence rūpa-tanhā for immaterial existence arūpa-tanhā D. 33
There are 18 'thought-channels of craving' tanhā-vicarita induced internally, and 18 induced externally; and as occurring in past, present and future, they total 108; see A. IV, 199; Vibh., Ch. 17 Khuddakavatthu-Vibhanga.
According to the dependent origination, craving is conditioned by feeling; on this see D. 22 section on the 2nd Truth.
Of craving for existence bhava-tanhā it is said A. X, 62:;No first beginning of the craving for existence can be perceived, o Bhikkhus, before which it was not and after which it came to be. But it can he perceived that craving for existence has its specific condition. I say, o Bhikkhus, that also craving for existence has its condition that feeds it sāharam and is not without it. And what is it? 'Ignorance', one has to reply.; - Craving for existence and ignorance are called;the outstanding causes that lead to happy and unhappy destinies courses of existence; see: Vis.M XVII, 36-42.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.