In the Dhammacakkappavattana Sutta, the Buddha taught:
"Dve me Bhikkhave anta pabbajitena na sevitabba”“Bhikkhus, one who has gone forth from the worldly life should not indulge in these two extreme parts (things, practices)”
"Katame dwe. Yo cayam kamesu kamasukhalikannuyogo – hino, gammo, pothujjaniko, anariyo, anatthasamhito. Yo cayam attakilamarthanuyogo – dukkha, anariyo, anattha-samhito."What are the two extreme things(parts, practises)?
Delighting in desirable sense-objects, pursuing and enjoying sensuous pleasures constitute one extreme practice. This practice is low or bad, being the habit of village and town folks; common and earthly, being indulged in by ordinary common worldlings, hence not pursued by the Noble Ones; not pertaining to the true interests one is seeking after. Such pursuit after sensuous pleasures is an extreme (part) practice which should be avoided.
The practice of self-mortification, which forms the other extreme practice results only in self-torture, infliction and suffering. It is not the practice of the Noble Ones, hence ignoble, unclean and does not tend to one’s own welfare and interest. This practice should also be avoided.
Extracted from The Great Discourse on the Wheel of Dhamma, translated by Venerable Mahasi Sayadaw
http://www.buddhanet.net/pdf_file/damachak.pdf