Adi Shankaracharya

Of the tree Samsara ignorance is the seed the identification with the body is its sprout attachment its tender leaves, work its water, the body its trunk, the vital forces its branches, the organs its twigs, the sense-objects its flowers, various miseries due to diverse works are its fruits, and the individual soul is the bird on it.  (Vivekachudamani 145)

This bondage of the non-Self springs from ignorance, is self-caused, and is described as without beginning and end. It subjects one to the long train of miseries such as birth, death, disease, and decrepitude. (Vivekachudamani 146)

There are three things that are rare indeed, a human birth, the longing for liberation (freedom from reincarnation), and the protecting care of a perfected sage. (Vivekachudamani 3)

Since all beings without exception are modifications of the essence of food and since all are equally descendents of Brahman, why is man alone unique? Because of his pre-eminence. In what again does this pre-eminence consist? In his competence for  karma (work) and  jnana (knowledge). For man alone is qualified for rights and duties as also for knowledge, by virtue of his ability, craving (for results) and non-difference (to results). Taittiriya Upanisad (II, I, 1) Commentary

Adi Shankaracharya