According to religious life, society is divided into four social divisions — brahmana, ksatriya, vaisya and sudra — and four spiritual divisions — brahmacarya, grhastha, vanaprastha and sannyasa. One needs to be trained to become a brahmana, ksatriya, vaisya or sudra, just as one is trained to become an engineer, doctor or lawyer. Those who are properly trained can be considered human beings; if one is not trained socially and spiritually — that is, if one is uneducated and unregulated — his life is on the animal platform. Among animals there is no question of spiritual advancement. Spiritual life can be attained by proper training-either by following the principles of varna and asrama or by being directly trained in the bhakti school by the methods of sravanam kirtanam visnoh smaranam pada-sevanam/ arcanam vandanam dasyam sakhyam atma-nivedanam [SB 7.5.23].

From Srila Prabhupada's purport to Madhya 24.94