Within the nature of reality in MahĀyĀna ontology, emptiness (śūnyatā) must be realized en route to enlightenment. The term śūnyatā has been glossed as "openness," "inconceivability," or "unlimitedness," but is best translated as "emptiness" or "voidness." It refers to what dharmas (elements of reality) really are through what they are not: not as they appear, not conceptualizable, not distinguishable, and, above all, lacking permanent, independent, intrinsic existence.
Although emptiness is sometimes mentioned in non-Mahāyāna texts, where it describes, for example, the contents of an advanced meditative state, the nonexistence of a self, or the absence of defilements in nirvĀṆa, the PrajÑĀpĀramitĀ literature of the Mahāyāna brought emptiness to prominence in Buddhist wisdom discourse. In paradoxical rhetoric, these sūtras describe emptiness as the true nature of all entities and concepts, from form through a buddha's awareness; thus, there really is no form, no buddha. This apparently nihilistic claim has been the subject of commentarial exegesis, philosophical disputation, meditative investigation, and ethical reflection throughout the Mahāyāna world. Still, emptiness is simply a radicalization and universalization of the earlier Buddhist idea of no-self (anatman), so that the view that there exists no unchanging subsisting person is extended to all possible objects and ideas, whether pure or impure, Buddhist or non-Buddhist—since grasping at true existence in any of them (including emptiness itself) will preclude the uprooting of defilements, hence the attainment of liberation and buddhahood.
In India, the most important philosophical reflection on emptiness emerged from the Madhyamaka school, beginning with NĀgĀrjuna (ca. second century c.e.), whose Madhyamakakārikā (Verses on Madhyamaka) uses reductive reasoning to demonstrate the untenability, hence emptiness, of various key concepts, including causation, time, and nirvāṇa. Nāgārjuna asserts, however, that emptiness is nihilistic only for those who ignore the distinction between two truths: the ultimate, in which everything truly lacks intrinsic existence; and the conventional, in which, precisely because they are empty (that is, interdependent), things exist and function, and concepts are valid. Subsequent Madhyamaka thinkers extended Nāgārjuna's analysis, reflecting on the implications of emptiness for such issues as the role of rationality on the path, the admissibility of syllogistic arguments "proving" emptiness, the "truth" value of conventional truths, the absoluteness of the negation involved in emptiness, the status of morality and compassion, the content of an awareness realizing emptiness, and the rapidity with which realization of emptiness effects enlightenment.
Other Mahāyānists analyzed emptiness, too. YogĀcĀra school writers agreed on its ultimacy, but described it as the absence of concepts in perfected awareness, or as an external object's inseparability from the consciousness perceiving it. Texts on tathĀgatagarbha (buddha-nature) sometimes implied that emptiness is different on different levels: Saṃsāric phenomena are empty of intrinsic existence, but buddha-awareness is empty of samsaric phenomena, itself being pure, permanent gnosis. In Tibet, these ideas were described as the intrinsic emptiness and extrinsic emptiness views, respectively. The Huayan jing (Avataṃsakasūtra; Flower Garland Sūtra) and East Asian schools based upon it, such as the Huayan school, portrayed emptiness as the perfect interpenetration of all phenomena. In tantric traditions, emptiness is the adamantine nature of reality, inseparable from a clear, blissful gnostic awareness; worlds and beings, maṆḌalas and deities, arise from and return to it, in reality as in meditative practice.
Discourse about emptiness was central to scholastic and meditative traditions in Tibet. It was also central to the philosophical treatises of the Sanlun, Huayan, and Tiantai schools of China, Korea, and Japan, and to the texts and praxis of East Asian Chan. Contemporary Buddhists, both Asian and Western, continue to explore the philosophical and practical implications of emptiness, reexamining traditional explanations of it, while aligning it with modern scientific and philosophical concepts, such as relativity, ecology, and deconstruction.
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Roger R. Jackson