In the Buddhist countries of southern Asia, there never arose any serious differences on the fundamentals of Buddhism. All these countries - Sri Lanka, Cambodia, Laos, Burma, Thailand, have accepted the principles of the Theravada school and any differences there might be between the various schools is restricted to minor matters.
The earliest available teachings of the Buddha are to be found in Pali literature and belongs to the school of the Theravadins, who may be called the most orthodox school of Buddhism. This school admits the human characteristics of the Buddha, and is characterised by a psychological understanding of human nature; and emphasises a meditative approach to the transformation of consciousness. The teaching of the Buddha according to this school is very plain. He asks us to 'abstain from all kinds of evil, to accumulate all that is good and to purify our mind'. These can be accomplished by The Three Trainings: the development of ethical conduct, meditation and insight-wisdom.
The philosophy of this school is that all worldly phenomena are subject to three characteristics - they are impermanent and transient; unsatisfactory and that there is nothing in them which can be called one's own, nothing substantial, nothing permanent. All compounded things are made up of two elements - the non-material part and the material part. They are further described as consisting of nothing but five constituent groups, namely the material quality, and the four non-material qualities - sensations, perception, mental formatives and consciousness. When that perfected state of insight is reached, i.e. Nibanna, that person is a 'worthy person' an Arhat. The life of the Arhat is the ideal of the followers of this school, a life where all (future) birth is at an end, where the holy life is fully achieved, where all that has to be done has been done, and there is no more returning to the worldly life'.