The origins of Hinduism can be traced to the Indus Valley civilization sometime between 4000 and 2500 BCE. Though believed by many to be a polytheistic religion, the basis of Hinduism is the belief in the unity of everything. This totality is called Brahman. The purpose of life is to realize that we are part of God and by doing so we can leave this plane of existance and rejoin with God. This enlightenment can only be achieved by going through cycles of birth, life and death known as samsara. One's progress towards enlightenment is measured by his karma. This is the accumulation of all one's good and bad deeds and this determines the person's next reincarnation. Selfless acts and thoughts as well as devotion to God help one to be reborn at a higher level. Bad acts and thoughts will cause one to be born at a lower level, as a person or even an animal. Hindus follow a strict caste system which determines the standing of each person. The caste one is born into is the result of the karma from their previous life. Only members of the highest caste, the brahmins, may perform the Hindu religious rituals and hold positions of authority within the temples.