Since Buddha did not give answers to many of the vital philosophical issues, his followers tried to find answers to such questions. These attempts, in turn, gave rise to different philosophical directions and schools. The differences of opinion and doctrines emerged immediately after the death of Buddha. It is believed that there existed thirty such Buddhist philosophical schools. But only four of them survived and are traditionally known. They are as follows:
The Vaibhasika School: The Vaibhasika derives its name from its exclusive emphasis on a particular commentary, the abhidhammamahavaibhasa of Abhidhamma. In the true spirit of the doctrine of dependent origination, the Vaibhasika holds that reality is pure flux and change. This school belongs to Hinayana. The Vaibhasika speaks about the existence of the mental and the non-mental realities. It teaches that we can really know the external entities and the world outside. This notion is known as bahya-pratyeka-vada. This school holds onto pluralism, realism, and nominalism. According to Vaibhasika, the world is in reality as it appears to us. The ultimate constituents (dharma) of reality are the same as those which make up the world of our empirical experience. Since they hold that the dharma is ultimate and independent of our consciousness, Vaibhasika is realism. Vaibhasika is pluralism as it asserts dharma as distinct and irreducible. For Vaibhasika, the reality is particular and is devoid of any universal unchanging entities. Thus Vaibhasika can be considered as nominalism too.
The Sautrantika School: The name Sautrantika, derived from the fact that it gives greater importance to the authority of the sutra-pitaka of the Pali Canon. This school also belongs to Hinayana Buddhism. The Sautrantika subscribes both the mental and the non-mental reality. The important difference between both these schools is that while the Vaibhasika maintains direct perception of the external objects, the Sautrantika holds the inference from the perceptions which are representations of external objects.
The second important difference between the Sautrantika and Vaibhasika is that unlike the Vaibhasika, the Sautrantika distinguishes between the world as it appears to us (phenomena) and the world as it is in itself (nomena). Here Sautrantika denies the absolute, ultimate, and independent ontological status of dharma. Another point of disagreement between the Sautrantika and the Vaibhasika is concerned with the nature of the relation between successive pointinstants of existence. For the Vaibhasika the past, present, and future are equally real. The reason is that the present, which is admittedly real, cannot be the effect of an unreal past and the cause of an unreal future. Contrary to this, the Vaibhasika’s point is that the point-instant which has no duration cannot causally bring about its succeeding point-instant. For, the cause and effect cannot be simultaneous.
The Yogacara School: This school belongs to Mahayana Buddhism. There are two different accounts of the origin of the name Yogacara of this school. According to one account, the followers of this school emphasized yoga (critical enquiry) along with acara (conduct). According to the other, the adherents of this school practiced yoga for the realization of the truth, that reality is of the nature of consciousness. The core of the doctrine of the Yogacara is that consciousness (mind) alone is ultimately real. Thus, external objects are regarded as unreal. For Yogacara, all internal and external objects are ideas of the mind. Thus, it is impossible to demonstrate the independent existence of external objects.
The philosophers of this school are known as the advocates of consciousness (vijnanavada). Yogacara offers another argument to deny the independent existence of the external object, which seems to be a criticism to the Sautrantika and the Vaibhasika. It argues that if there is an object outside, it must be indivisible, partless, and atomic, or divisible and composite. If it is the former, it cannot be perceived since atoms are too minute. If it is composite, we cannot perceive all the parts and the sides of the object simultaneously. Thus, in either case the existence of the external objects is denied. Another important argument against the existence of the external objects is based on the doctrine of momentoriness. The Yogacara points out that, since objects are not substances but duration- less point instants, it is difficult to understand how a momentary object can be the cause of consciousness. If it is the cause of consciousness, there must be a time lapse between the arising of the object and our consciousness of it.
The Madhaymika School: This school also belongs to Mahayana Buddhism. The literal meaning of the term Madhaymika is ‘the farer of the Middle Way’. The Madhaymika avoids all the extremes, such as, eternalism and annihilationism, self and non-self, matter and spirit, unity and plurality, and identity and difference. The founder of this school is supposed to be Nagarjuna of the second century CE. Aryadeva, Candrakirti, Kumarajiva, and Santideva are the other prominent figures of this school. One of the most important insights of Nagarjuna is the origin and nature of philosophy and philosophical conflicts. For him, knowledge is the means by which man seeks to unite the self and the other. Knowledge is propositional, and propositions are constituted of concepts, and concepts refer names (nama) and forms (rupa). Hence, the reality which philosophers create in their knowledge is the reality of names and forms, and not reality as it is in itself. The Madhaymika claims that concepts and conceptual systems are relative to each other. They cannot stand by themselves and generate truths. Consequently, no system can claim absolute truth and validity. The truth of each system can be relative and partial. Nagarjuna teaches that it is absurd to speak about reality as true or false. Reality simply is. The emptiness (Sunyata) of concepts does not entail the emptiness of the reality. What he means by Sunyata is not that reality is nonexistent or illusory, but only that it is devoid of any entities which we think. The Madhaymika claim that unlike the other three schools, their philosophy is very close to the teachings of Buddha. The notion of relativity and sunyata (emptiness) are none other than the doctrine of dependent origination which Buddha emphasized.
Nagarjuna brings the notion of two types of truths: the lower truth and higher truth. He calls the phenomenal truths as lower truth, since we cannot find any absolute truth in this world. All phenomenal truths are relative, conditioned, and valid within particular domains of our perceptual-conceptual experience. The higher truth is beyond percepts and concepts, ineffable and defying all descriptions. It is absolute, supramundane, and unconditional. It is grasped through intuitive insights.