The early history of Buddhist thought in India can better be characterized as a shift from primarily moral and religious teachings concerned with the attainment of enlightenment to a more comprehensive system of thought, in the midst of which philosophical schools and systems gradually came into existence.
This process is wound up with internal dynamics of the Buddhist religion as well as with more general developments in ancient Indian intellectual environs. After the death of the Buddha, attempts were made to gather his teachings and transmit them in a commonly agreed form, first orally, then also in writing (The Tripitaka). In addition to collecting the Buddha's speeches and rules for monastic life (Vinaya), monks soon undertook it to condense what they considered the essential elements of Buddhist doctrine into lists of categories, provided with extensive commentary. This process took shape from about the 2nd century BC to probably the 2nd century AD.
Buddhist philosophy must be seen in the context of the mature Vedic system that existed in India at the time of the Buddha. The Vedas address the question of knowledge and methods of achieving it. The different ways of obtaining meaning form part of the six schools of Hindu philosophy.
At some point, philosophical activity seems to have turned towards categorizing all that exists, towards providing a set of categories into which all possible entities or objects could be subsumed. In Buddhism, the ancient concept of a "constituent factor" (Sanskrit dharma), perhaps originally conceived of as a constituent factor of Buddhist doctrine, came to fulfill this role of an ontological category, in the sense of a constituent factor of mind as well as matter.
This increasing focus on questions of ontology, of existence, led to a reshaping of older teachings that had primarily concentrated on ethics and liberation. Questions to which the Buddha is reported to have refused an answer, such as whether a soul exists, whether it is permanent, or whether it is identical with or different from the body, now came to be addressed. Scholars have different views about what is probably the most famous teaching of Buddhism, that of anatta or "not-self"; it appears plausible to interpret older, canonical teachings as aiming mainly at a supression or neglect of clinging to the notion of an eternal, blissful and world-removed self, because such clinging would stand in the way of realizing universal impermanence and inherent suffering, necessary preconditions for attaining enlightenment. These teachings do not address the issue of whether a soul exists; such questions are often dismissed as fruitless speculations.
The philosophical schools of Buddhism, amongst which the so-called Sarvastivada school is the most important historically in this context, soon came to furnish the teaching that bodily or mental factors are not the self with the additional connotation "because no self exists as a permanent substrate of mental states". History of logic