The philosophical traditions of India have always honoured logic, the art of reasoning. Hindu, Buddhist and Jain sages alike look beyond the limits of reason and conceptualization, but they insist upon the crucial role played by clarity of thinking in self-transcendence and spiritual realization. Like Plato and unlike Aristotle, Indian thinkers paid more attention to the logic of concepts and principles of inference than to the topology of categories. Nagarjuna raised logic to the recondite level of compelling dialectic, and those who came after him refined various aspects of his crucial work. Vasubandhu in particular addressed the need for precise dialectic, because debate was neither a mere intellectual game nor an occasion for mutual hostilities. The spiritual seriousness of argumentation is evidenced by the fact that an individual defeated in a thorough and exhaustive exchange was honour-bound to accept the victor's standpoint, even if this meant changing religious allegiances. Whilst logic had been a dominant concern of many thinkers and teachers, Dignaga turned his whole mind to the topic and refounded the art of reasoning in the service of Truth.
Sometime in the early years of the fifth century, Dignaga (also known as Dinnaga) was born into a Brahmin family in Simhavakta near Kanchi (Kanchipuram). Nothing is known of his early years except that he took as his Upadhyaya, spiritual preceptor, Nagadatta of the Vatsiputriya school. This branch of Buddhist thought espoused the view that there existed a kind of real personality independent of the elements or aggregates composing it. Though Dignaga learnt the whole of the Tripitaka, the Three Baskets of non-Mahayana Buddhist wisdom, he was not satisfied with Vatsiputriyan attempts to deny the eternality of self whilst affirming its continuity through a series of lives. When his preceptor enjoined him to search for the "indescribable self", the principle of 'I' which is neither identical with nor different from the skandhas, he opened all the windows of his dwelling by day and filled every corner of it with lamps by night. Stripping himself naked, he repeatedly examined himself from every angle. Soon other monks noticed his bizarre behaviour and reported it to Nagadatta. When asked why he carried on in this way, he replied that he failed to understand what he had been instructed to seek and so had bared himself to see if it might be uncovered. Recognizing a challenge to his doctrine, Nagadatta grew angry and ordered Dignaga to leave his vihara or monastic community.
Dignaga left in silence. He knew that he could demonstrate the untenability of Nagadatta's doctrines, but he was profoundly aware of the wrongness of attacking the teachings of one's Upadhyaya. His efforts to indicate a fundamental problem indirectly had only angered Nagadatta, and Dignaga departed with a heavy heart. In time, however, he encountered Vasubandhu, who fully understood Dignaga's insights, and willingly undertook to teach him Yogachara thought. Vasubandhu explained five hundred sutras to Dignaga, including all those belonging to the Mahayana and Hinayana traditions, as well as the mysterious dharani sutras. He mastered the science of vidya mantra and refined the art of debate. Nonetheless, he felt that he was a worthless student because of his inability to persuade his first teacher to reconsider his doctrines. Once when he was in a state of spiritual depression, Manjushri, the sword-wielding Bodhisattva of supernal wisdom, appeared to him in a vision, brought him to his senses and instructed him at great length in the Dharma. Dignaga was spiritually regenerated and took up his work with a zeal that never waned for the rest of his life. He retired to a cave in the Bhotashela hill near Odivisha (Orissa), where his intense meditation bore the inexpressible fruit of samadhi.
A few years later, a great debate was arranged at Nalanda. As in most monastic communities, Buddhists of diverse schools dwelt peacefully together. Even while they vigorously debated the merits of their differing doctrines, they were bound together in the brotherhood of the Dharma. Nalanda not only hosted every shade and hue of Buddhist thought, but also housed numerous non-Buddhist teachers and students. Dignaga was invited to debate a group of remarkable tirthikas, non-Buddhist instructors, renowned for their dialectical agility. Dignaga defeated each one in turn so decisively that they all joined the Sangha. He tarried for some time at Nalanda, where he taught the sutras and wrote many volumes on Yogachara doctrine and on logic. Eventually he returned to his cave near Odivisha and devoted himself to meditation. While there, he resolved to compose the Pramanasamuccaya, aphorisms on pramana, valid knowledge.
According to Taranatha, he wrote the beginning lines of this great treatise with a piece of chalk on a rock. When he took his begging bowl and went out for his morning round, a Brahmin called Krishnamuniraja came upon these words, realized their portentous significance, and erased them. The next morning Dignaga began again, but once more the words were eradicated in his absence. On the third morning Dignaga wrote the same words, but added the sentence "Know this to be extremely important" and challenged the defacer to a debate. Reading this challenge, Krishnamuniraja sat down and waited for Dignaga to return from gathering alms. When Dignaga came back to his dwelling, the two engaged in a ferocious debate. When Dignaga emerged victorious, he invited the tirthika to accept the Dharma, but Krishnamuniraja threw a magic powder which ignited the belongings of Dignaga, and then fled. Once again, Dignaga fell into a spiritual depression, blaming himself for failing to convert his opponent. But Manjushri appeared to him and remonstrated with him, warning that "wrong ideas result from evil company". Assuring him that no one would bring harm to his treatise, Manjushri promised to remain Dignaga's kalyana-mitra, true spiritual friend, until he attained full enlightenment.
Dignaga composed his treatise in peace and returned to his meditations. Once he grew ill. After his mendicant rounds, he returned to the forest and fell fast asleep. While he slept, he had visions of numerous glorious Buddhas who instructed him. Meanwhile, the king, out in the forest for recreation, came across the sleeping monk. Celestial deities were raining flowers upon Dignaga, the forest plants were bowing in his direction, and elephants stood quietly over him to afford him shade. When the king awoke him with the sweet sounds of musical instruments, he asked, "Are you Dignaga?" "So I am called", the monk replied. And the king prostrated before his feet. Tradition records that Dignaga journeyed south and restored damaged viharas. The king of Odivisha and the royal treasurer helped build monasteries, and Dignaga continued to draw opponents into the embrace of the Dharma. He performed a number of wondrous feats, including restoring a precious tree to life by chanting over it. Though he had many followers, he refused to have attendants, preferring to live alone. He died alone in the verdant forests of Odivisha, having lived a life dedicated to meditation and teaching others to clarify consciousness for the sake of Truth.
Dignaga set himself a monumental task. He sought on the one hand to produce a definitive treatise on the principles of logic which could be used to understand the fundamental meaning of ideas and ideals. On the other hand, he wanted to show that diverse standpoints could be understood, communicated, assessed and reconciled. Some resisted his teaching, for it demanded rigorous mental training, the renunciation of bias and a desire to bring all one's mental faculties to peak awareness. Some thought he was a quibbler and a hair-splitter, but such judgements confuse the careful, even ponderous, quality of his work with the fiery spirit which infuses it. Despite the stringent dialectical requirements of the Pramanasamuccaya, it rapidly assumed importance for Buddhists of every school, and Hindus, Jains and Zoroastrians alike felt the need to grasp its contents not only for debates like those held at Nalanda but also as an aid in understanding their own spiritual inheritance. It was translated into Chinese in the second half of the sixth century, and it remains the foundation stone of the 'new logic', fulfilling the prophecy of Manjushri that "in later times this Shastra will become the sole eye of all the Shastras".
The first chapter of the Pramanasamuccaya takes up the question of pramana, valid knowledge. A number of dialecticians treated perception, logical inference, the testimony of others, authority and scripture as independent means to truth and implicitly endowed them with equal epistemological status. Dignaga did not question the value of these sources of knowledge and opinions, but he clarified their relationship to one another and delineated their ranges of application. For him there are only two means to knowledge – pratyaksha and anumana, perception and inference. Dignaga offered no definition of perception, in part because it is common knowledge and in part because it cannot be defined save through itself. Pure perception is free from preconception of any kind and is therefore unconnected with name, genus, species and conceptualization. This suggests that most of what is ordinarily called perception is in fact tainted by the mental constructs and habits which mediate and obscure the function of the senses. Thus, when one mistakes a rope for a snake, the error does not occur because of misperception but rather because of preconception based on fear, previous experience or memories of frightening encounters recounted by others in the past. Given this characterization of perception, it is possible for the man of meditation to perceive supramundane realms of objects and planes of being inaccessible to untrained consciousness victimized by the kleshas and caittas enumerated by Vasubandhu.
Perception, never susceptible to categorization, is in every case unique. Thus perception is invariably of individual characteristics or "infinite peculiarities". In attempting to convey to another what one has perceived, one invokes class descriptions, communicating a generalized phenomenon – a cow, book or whatever – rather than the unique perception one experienced. Inference, however, is quite another matter, for inferential knowledge is general and can be expressed by name, genus, species and all the categories of thought and language. Dignaga cannot say that perception involves the interaction of senses with their objects, because such a claim could be established only by invoking the elements of inference. Thus, Dignaga's conception of perception is the epistemological correlate of chittamatra, Mind-only. Those pairs of opposites which give value and tone to perceptions – pleasure and pain, for example – are not objects of knowledge but rather colorations of consciousness.
Anumana, inference, is of two kinds: svarthanumana and pararthanumana, inference for oneself and inference for another. Inference for oneself is knowledge of a thing derived from its distinctive marks or characteristics. The mark might be the effect of the thing inferred, as one infers fire from smoke which is its effect. It might be essential identity, as an acacia is identical with a tree. And it might be absence of perception signifying the non-existence of what is not perceived. Non-perception of a pot, for instance, permits the inference that no pot exists here where one is looking. Pararthanumana, inference for the sake of others, is more complex, for it is concerned with what one can demonstrate to another on the basis of what one can infer for oneself. In addition to the inference, one must show its validity by some parallel example to another. For instance, in the assertion that
the reason for the non-eternality of this sound is that it is the result of effort. The parallel example or homologue is a pot, which is also the product of effort, and the non-parallel example or heterologue is Akasha, which is not the product of effort.
Dignaga devised a list of nine reasons which can link subject and predicate, centering on non-eternality, being produced or a product of something else, audibility and tangibility. Depending on whether the reason is wholly present in, partially present in or absent from the homologue and the heterologue, Dignaga created a table of validity, which was the first attempt of its kind to systematize logical inferences. Though its structure is logical, the nature of his reasons requires basic knowledge of the world in order to determine the truth of arguments. As in Aristotelian syllogisms and in modern propositional logic, the conclusions are true if and only if the premisses are true and the logical form is valid. The reason (hetu) can be either affirmative or negative. It is affirmative when it is always accompanied by whatever is given in the predicate: the hill is fiery because it is smoky, smoke being the reason. It is negative when what is declared absent in the reason is absent in the predicate: the hill is not smoky because it is not fiery. Dignaga's "law of extension" requires that one who wishes to convince another of his own conclusion has to state both subject and predicate as well as the reason which connects them, along with suitable examples which illustrate the linkage between the reason and the predicate.
For Dignaga, comparison is not an independent source of knowledge, because recognizing the similarity between two objects is an act of perception. Similarly, the testimony of others depends upon either the credibility of the individual or the credibility of the fact itself. In the first case, inference is involved, whilst the second is a case of perception. Even though Dignaga reduced the means to knowledge to only two, he showed that the traditionally accepted sources of understanding do not have to be rejected, since they can be understood as special instances of perception and inference. He also discussed the importance of the use of analogy and sought to distinguish between acceptable and far-fetched analogues. The basic schema of Dignaga's system of logic left a variety of issues unresolved, many of which he took up in other treatises. In addition to extensive discussions of subjects and predicates (minor and major terms) and the use of example, he compiled illustrated collections of fallacies. He also warned against theses or propositions which one must reject out of hand because they are incompatible with perception, contradict inference, reject overwhelming public opinion, deny one's belief, are self-contradictory (e.g., "My mother has always been barren"), or use terms incomprehensible to the system of thought under discussion. He added that one cannot prove a thesis which is universally accepted (for example, "Fire is warm") precisely because of its universal acceptability.
Dignaga established a three-step method of proof when one reasons with oneself, and a five-step method for convincing others. Since all that one experiences short of Enlightenment is chittamatra, merely consciousness, clarity of thought is crucial at every stage of the Bodhisattva Path. For Dignaga, logic is not an end in itself but rather an invaluable aid in transforming the processes of consciousness. He elevated debate out of the murky plane of polemic into the realm of dispassionate discourse and became the founder of medieval Buddhist logic. In later centuries he was given the name Tarkapungava, Fighting Bull, for his formidable dialectical skills. Unfortunately, subsequent generations lost something of the sacred vision that brought Dignaga face to face with Manjushri and retained only the logic. Thus his pioneering work became the backbone of an uninspiring scholasticism and the tool of those who take greater delight in quibbling than in meditation. Nonetheless, his work was a valuable aid not just for the Yogachara school, but for all Buddhists and numerous Hindu traditions as well. His efforts have been honoured by the fact that all who have known them have used them freely.