本文就佛陀時代印度的六宗外道之教義追本溯源。這六宗外道是Shramana宗，生存於居領導地位的婆羅門教之外。這外道六師是撥無因果的Purana Kashyapa；Ajivikas的教主Maskari Goshaliputra；路伽耶陀（Lokayatas）的領導者Ajita Kesalakambin；大種論的信仰者Kakuda Katyayana；悲觀論者Sanjayin Vairattiputra；奢那教的領導人尼健子（Nirgrantha Jnatiputra），又名Mahavira。
這些外道包括由Ajivikas的宿命論，路伽耶陀的唯物主義論，到極端懷疑主義的Sanjayin Vairattiputra的弟子們。其他像奢那教，是很融合的一宗哲學。這些外道，有一些要麼相信來生，要麼乾脆否定。戒因問題與善惡之報的問題，似乎對於佛陀時代的人是一個一再出現的問題。大部份的這些外道否認守不守道德有什麼後果，或有什麼意義。因此大部份都拋棄業律與因果律，至少是在道德範疇是如此。一些外道師如Ajita Kesalakambin和Kakuda Katyayana教四或七大種，包涵整個精神宇宙。在這個物與人的主題裡，他們的行動與精神過程是會飄逝的，也不重要。唯一真實與永恆的實體是大種，或是極微塵粒，周遍自然現象之中。
This essay has been an effort to trace the teachings of the six major heterodox philosophical schools in India during the time of the Buddha. All the six schools discussed in this paper were shramana schools that existed outside the dominant Brahmanical religion. The teachers discussed in this paper are Purana Kashyapa, who denied moral causation; Maskari Goshaliputra, leader of the Ajivikas; Ajita Kesalakambin, leader of the Lokayatas; Kakuda Katyayana, a believer in elements; Sanjayin Vairatiputra, leader of the skeptics; and Nirgrantha Jnatiputra, alias Mahavira, leader of the Jainas.
The doctrines of these schools ranged from the fatalism of the Ajivikas and the materialism of the Lokayatas to the extreme skepticism of the followers of teachers such as Sanjayin Vairatiputra. Others, like the Jainas, were highly syncretistic in their philosophy. Some of these schools believed in rebirth, others denied it flatly. The question of moral causation and the validity of good and evil deeds seems to have a recurrent problem to the contemporaries of the Buddha. Most of these schools denied that moral or immoral conduct had any consequences, any meaning whatsoever. Thus most of them rejected karma and the principle of cause and effect, at least in the moral sphere. Some teachers like Ajita Kesalakambin and Kakuda Katyayana taught theories of four or seven elements that constituted the entire psychophysical universe. In this scheme of things people, their actions and mental processes were passing and insignificant. The only real, eternal entities were the elements, or extremely fine particles, that constituted all phenomena.
The greater part of this study is devoted to contrasting basic Buddhist teachings with the heterodoxies of the six schools. From the Buddhist point of view, adherents to these six schools fall into various extreme, erroneous views—either existence or non-existence, either eternalism or annihilationism, either spontaneity or deviant causation. In order to set the Buddhist refutations of the heterodox teachings into context, a general outline of key Buddhist concepts has been presented, concentrating on those features that set Bud-dhism apart from the six schools. These concepts include Buddhist theory of knowledge; the importance of moral conduct and the precepts as a basis for enlightenment; moral cause and effect and karma; the uniquely Buddhist doctrine of dependent origination; and the doctrines of no self and emptiness.
The last part of this paper outlines the refutations of the heterodox teachings in specific Mahayana Sutras, especially the Shurangama Sutra. The Buddhist view of elements is also touched on. The doctrine of the Treasury of the Thus Come One is briefly introduced, as it is presented in the Shurangama Sutra as transcending the Buddha's earlier teachings of causes and conditions. In the last section, the Bodhisattva's range of practice is delineated, as outlined in chapter 14 of the Dharma Flower (Lotus) Sutra.
In the 2500 years since the Buddha's parinirvana Buddhism has spread far outside of India and developed into numerous sects and schools, whose diverse teachings may appear difficult to reconcile with each other. But however elaborate the scholarly systems have become, the core teachings have remained intact. Doctrines such as dependent origination and no self have been continuously explained by both Theravada and Mahayana teachers. This study draws freely from canonical sources of both the Pali and the Mahayana canon, and is intended to be an introduction to Buddhist philosophy that emphasizes the basic unity of Buddhism. The Buddha's teachings are multilayered and inexhaustible, and even though there are both provisional and actual teachings, none of the Dharma taught by the Buddha should be slighted. All his innumerable teachings serve a purpose and are expediently devised to take across living beings of various capacities. Dogmatism and sectarian squabblings can best be avoided if the student of Buddhism always keeps in mind that the Dharma is a raft— something beneficial, something to be used and shared, but never clung to.