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Conversation with Distinguished Professor Graham Priest on Indian Logics, Nāgārjuna, Gottlob Frege and Peter Geach, and the PNC and the PEM: Distinguished Professor, Philosophy, City University of New York (4)


Author(s): Scott Douglas Jacobsen

Publication (Outlet/Website): In-Sight: Independent Interview-Based Journal

Publication Date (yyyy/mm/dd): 2021/06/22


From the professional website for Professor Priest: “Graham Priest grew up as a working class kid in South London. He read mathematics and (and a little bit of logic) at St. John’s College, Cambridge. He obtained his doctorate in mathematics at the London School of Economics. By that time, he had come to the conclusion that philosophy was more fun than mathematics. So, luckily, he got his first job (in 1974) in a philosophy department, as a temporary lecturer in the Department of Logic and Metaphysics at the University of St Andrews. The first permanent job he was offered was at the University of Western Australia. He moved to Australia when he took up the position, and has spent most of his working life there. After 12 years at the University of Western Australia, he moved to take up the chair of philosophy at the University of Queensland, and after 12 years there, he moved again to take up the Boyce Gibson Chair of Philosophy at Melbourne University, where he is now emeritus. While he was there, he was a Fellow of Ormond College. During the Melbourne years, he was also an Arché Professorial Fellow at the University of St Andrews. He is a past president of the Australasian Association for Logic, and the Australasian Association of Philosophy, of which he was Chair of Council for 13 years. He was elected a Fellow of the Australian Academy of Humanities in 1995, and awarded a Doctor of Letters by the University of Melbourne in 2002. In 2009 he took up the position of Distinguished Professor at the Graduate Center, City University of New York, where he now lives and works. Graham has published in nearly every leading logic and philosophy journal. At the last count, he had published about 240 papers. He has also published six monographs (mostly with Oxford University Press), as well as a number of edited collections. Much of his work has been in logic, especially non-classical logic, and related areas. He is perhaps best know for his work on dialetheism, the view that some contradictions are true. However, he has also published widely in many other areas, such as metaphysics, Buddhist philosophy, and the history of philosophy, both East and West. Graham has travelled widely, lecturing and addressing conferences in every continent except Antarctica. For many years, he practiced karatedo. He is a third dan in Shobukai, and a fourth dan in Shitoryu (awarded by the head of style, Sensei Mabuni Kenei in Osaka, when he was training there). Before he left Australia he was an Australian National kumite referee  and kata judge. Nowadays, he swims and practices taichi. He loves (good operajazz , and 60s rock … and East Asian art.” He discusses: Indian logics; Buddhist tradition; against dialetheism; Gottlob Frege and Peter Geach; the traditional splits between realists and the anti-realists, and the idealists and the constructivist; most significant developments from this challenge to the Aristotelian logic; upcoming projects.

Keywords: Dialetheism, Gottlob Frege, Graham Priest, logicians, Nāgārjuna, paraconsistency, Peter Geach, philosophy, Principle of the Excluded Middle, Principle of Non-Contradiction.

Conversation with Distinguished Professor Graham Priest on Indian Logics, Nāgārjuna, Gottlob Frege and Peter Geach, and the PNC and the PEM: Distinguished Professor, Philosophy, City University of New York (4)

*Please see the footnotes, bibliography, and citation style listing after the interview.*

Scott Douglas Jacobsen: In ancient Indian logics, there were four possibilities: true, false, both true and false, and neither true nor false. How does this mesh well with the dialethetic approach to knowing?

Professor Graham Priest[1],[2]: Well, it’s not quite as simple as that. Views on the truth values differed an Ancient India. The Hindu Nyāyā philosophers held that there were just two (true and false), as did some of the later Buddhist philosophers, such as Dharmakīrti and Dignāga. Earlier Buddhist philosophers held that there were four (true, false, both, and neither). (This is the catuṣkoṭi.) Jain philosophers held that there were seven! (This was the saptabhaṅgī.) One of these was indescribable. Whether this was a form of both or neither is unclear.

However, any view which holds that some things are both true and false accommodates dialetheism.

Jacobsen: The Mahayana Buddhist tradition through Nagarjuna states all is real and not real, both real and not real, and neither real nor not real. Why is this a statement about the world rather than a mystical incantation about some ineffable property?

Priest: Well, interpreting Nāgārjuna is a vexed question—both within Buddhist traditions and amongst modern scholars. And it is not clear how this passage is best interpreted. But it is standard in Buddhist thought that there are two realities (satyas). There is the conventional reality of the world as we actually experience it, and the ultimate reality of how things actually are. Probably the best interpretation of this passage is that some thing can be real (conventionally), not real (ultimately), both real (conventionally) and not real (ultimately), and neither real (ultimately) nor not real (conventionally). All rather mundane, I’m afraid.

Jacobsen: Why are arguments against dialetheism focused on negations? Any examples?

Priest: Well, a dialetheia is a pair of statements of the form A and ~A. There isn’t much to focus on except negation! Some people have argued that the truth of ~A rules out the truth of A, as a matter of definition. Clearly, such an argument is question-begging and worthless.

Jacobsen: How would you properly respond to them?

Priest: See the previous question. More generally, how negation works and what properties it has been controversial throughout the history of Western philosophy/logic—and no more than now.

Jacobsen: Gottlob Frege and Peter Geach argued, I think, the rejection of X means the acceptance of ~X. Why is the adjoining not necessarily correct or so obvious to bring together?

Priest: Yes, they did, though note that rejection is not the same as negation. Negation is on operation that applies to a sentence and delivers a sentence with different content. Assertion and denial are actions you perform with a sentence. But even without worrying about dialetheism, the Frege/Geach view is clearly false. One often finds that one’s views are inconsistent in a way that one hadn’t realised. (It’s common in a discussion of any complexity.) One discovers that one asserts A and ~A. In the second assertion, one is clearly not denying A. That is, after all, what one accepts (until one changes one’s views). Once one brings dialetheism into the picture, matters become even more obvious. I assert that the liar sentence is true and I assert that the liar sentence is not true. In the latter assertion I am obviously not denying that it is true; for that is exactly what I think it is.

Jacobsen: Given some of the prior commentary, how do paraconsistent thinkers – ahem – think about the traditional splits between realists and the anti-realists, and the idealists and the constructivist?

Priest: Well, dialetheism is neutral on this question. It says that some contradictions are true. It says nothing about whether truth is to be construed realistically or anti-realistically. As far as paraconsistent logic goes, there are many of these. Perhaps some are more realism-friendly, and some are more antirealism-friendly—though this depends on how you think these metaphysical views play out logically. Thus, for example, suppose you think (misguidedly, I believe) that the Principle of Excluded Middle is characteristic of realism, and its failure is characteristic of anti-realism. There are both paraconsistent logics with and paraconsistent logics without the PEM.

Jacobsen: What have been the most significant developments from this challenge to the Aristotelian logic notions or interpretations of meaningfulness, validity, rationality, and truth?

Priest: Well, oddly enough, this changes virtually everything, and virtually nothing. Because the PNC is a principle that has tightly circumscribed nearly everything in Western philosophy, removing it opens up a wide vista of new possible positions on nearly everything; from metaphysics to ethics to the philosophy of mind, to philosophical hermeneutics, to aesthetics. Actually, I think this is a rather exciting development in philosophy.

On the other hand, one thing we have learned is that giving up the PNC changes virtually nothing. All the old theories of meaning are still possible; validity can still be defined in terms of truth preservation, or specified in terms of a bunch of rules of inference; to be rational is still to ‘apportion your beliefs according to the evidence’ (as Hume put it); and all the old theories of truth are still available and viable—to whatever extent they were before. In a way, this all makes it even more puzzling as to why so many philosophers have felt that the sky would fall in if some contradictions were permissible.

Jacobsen: Finally, any upcoming projects or books, or other paraconsistent philosophers for others to look into – plug, plug?

Priest: The book I currently in the process of writing is on socio-political philosophy, and has nothing to do with paraconsistency or dialetheism (or, more generally logic and metaphysics). After that, I have in mind a book on nothingness, which will certainly deploy paraconsistency and dialetheism.

There is so much written on paraconsistent logic now, that it is a long time since I have been able to keep track of it all. Even the literature on dialetheism is now so large that I’m not aware of all the things that are happening. So let me just flag one book in press. This is coming out with Oxford University Press, hopefully later this year. It’s written by Yasuo Deguchi, Jay Garfield, Bob Sharf, and myself. It’s called Whereof one Cannot Speak, and it’s on dialetheism in East Asian philosophy—mainly Chinese and Japanese Buddhism.

Jacobsen: Thank you for the opportunity and your time, Professor Priest.

Priest: You’re welcome.

Appendix I: Footnotes

[1] Distinguished Professor, Philosophy, Graduate Center, City University of New York (2009-Present).

[2] Individual Publication Date: June 22, 2021:; Full Issue Publication Date: September 1, 2021:


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