In brief, the principle of dependent origination can be understood in the following three ways. First, all conditioned things and events in the world come into being only as a result of the interaction of causes and conditions. They don't just arise from nowhere, fully formed. Second, there is mutual dependence between parts and the whole; without parts there can be no whole, without a whole it makes no sense to speak of parts. This interdependence of parts and the whole applies in both spatial and temporal terms. Third, anything that exists and has an identity does so only within the total network of everything that has a possible or potential relation to it. No phenomenon exists with an independent or intrinsic identity.
And the world is made up of a network of complex interrelations. We cannot speak of the reality of a discrete entity outside the context of its range of interrelations with its environment and other phenomena, including language, concepts, and other conventions. Thus, there are no subjects without the objects by which they are defined, there are no objects without subjects to apprehend them, there are no doers without things done. There is no chair without legs, a seat, a back, wood, nails, the floor on which it rests, the walls that define the room it's in, the people who constructed it, and the individuals who agree to call it a chair and recognize it as something to sit on. Not only is the existence of things and events utterly contingent but, according to this principle, their very identities are thoroughly dependent upon others.
In physics, the deeply interdependent nature of reality has been brought into sharp focus by the so-called EPR paradox - named after its creators, Albert Einstein, Boris Podolsky, and Nathan Rosen - which was originally formulated to challenge quantum mechanics. Say a pair of particles is created and then separates, moving away from each other in opposite directions - perhaps to greatly distant locations, for example, Dharamsala, where I live, and say,
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Another significant set of issues in quantum mechanics concerns the question of measurement. I gather that, in fact, there is an entire area of research dedicated to this matter. Many scientists say that the act of measurement causes the "collapse" of either the wave or the particle function, depending upon the system of measurement used in the experiment; only upon measurement does the potential become actual. Yet we live in a world of everyday objects. So the question is, How, from the point of view of physics, do we reconcile our commonsense notions of an everyday world of objects and their properties on the one hand and the bizarre world of quantum mechanics on the other? Can these two perspectives be reconciled at all? Are we condemned to live with what is apparently a schizophrenic view of the world?
At a two-day retreat on the epistemological issues pertaining to the foundations of quantum mechanics and Buddhist Middle Way philosophy at Innsbruck, where Anton Zeilinger, Arthur Zajonc, and I met for a dialogue, Anton told me that a well-known colleague of his once remarked that most quantum physicists relate to their field in a schizophrenic manner. When they are in the laboratory and play around with things, they are realists. They talk about photons and electrons going here and there. However, the moment you switch into philosophical discussion and ask them about the foundation of quantum mechanics, most would say that nothing really exists without the apparatus defining it.
Somewhat parallel problems arose in Buddhist philosophy in relation to the disparity between our commonsense view of the world and the perspective suggested by Nagarjuna's philosophy of emptiness. Nagarjuna invoked the notion of two truths, the "conventional" and the "ultimate," relating respectively to the everyday world of experience and to things and events in their ultimate mode of being, that is, on the level of emptiness. On the conventional level, we can speak of a pluralistic world of things and events with distinct identities and causation. This is the realm where we can also expect the laws of cause and effect, and the laws of logic such as the principles of identity, contradiction, and the law of the excluded middle-to operate without violation. This world of empirical experience is not an illusion, nor is it unreal. It is real in that we experience it. A grain of barley does produce a barley sprout, which can eventually yield a barley crop. Taking a poison can cause one's death and, similarly, taking a medication can cure an illness. However, from the perspective of the ultimate truth, things and events do not possess discrete, independent realities. Their ultimate ontological status is "empty" in that nothing possesses any kind of essence or intrinsic being.
I can envision something similar to this principle of two truths applying in physics. For instance, we can say that the Newtonian model is an excellent one for the commonsense world as we know it, while Einsteinian relativity-based on radically different presuppositions-represents in addition an excellent model for a different or more inclusive domain. The Einsteinian model describes aspects of reality for which the states of relative motion are crucial but does not really affect our commonsense picture under most circumstances. Likewise, the quantum physics models of reality represent the workings of a different domain-the mostly "inferred" reality of particles, especially in the arena of the microscopic. Each of these pictures is excellent in its own right and for the purposes for which it was designed, but if we believe any of these models to be constituted by intrinsically real things, we are bound to be disappointed.
Here I find it helpful to reflect on a critical distinction drawn by Chandrakirti (seventh century C.E.) in relation to the domains of discourse that pertain to the conventional and the ultimate truths of things. Chandrakirti argues that, when formulating one's understanding of reality, one must be sensitive to the scope and parameters of the specific mode of inquiry. For example, he argues that to reject distinct identity, causation, and origination within the everyday world, as some interpreters of the philosophy of emptiness had suggested, simply because these notions are untenable from the perspective of ultimate reality, constitutes a methodological error.
On a conventional level, we see cause and effect all the time. When we're trying to find who's at fault in an accident, we are not delving into the deeper nature of reality, where an infinite chain of events would make it impossible to place blame. When we accord such characteristics as cause and effect to the empirical world, we are not working on the basis of a metaphysical analysis that probes the ultimate ontological status of things and their properties. We do so within the boundaries of everyday convention, language, and logic. In contrast, Chandrakirti argues, the metaphysical postulates of philosophical schools, such as the concept of the Creator or the eternal soul, can be negated through the analysis of their ultimate ontological status. This is because these entities are posited on the basis of an exploration into the ultimate mode of being of things.
In essence, Nagarjuna and Chandrakirti are suggesting this: when we relate to the empirical world of experience, so long as we do not invest things with independent, intrinsic existence, notions of causation, identity, and difference, and the principles of logic will continue to remain tenable. However, their validity is limited to the relative framework of conventional truth. Seeking to ground notions such as identity, existence, and causation in an objective, independent existence is transgressing the bounds of logic, language, and convention. We do not need to postulate the objective, independent existence of things, since we can accord robust, nonarbitrary reality to things and events that not only support everyday functions but also provide a firm basis for ethics and spiritual activity. The world, according to the philosophy of emptiness, is constituted by a web of dependently originating and interconnected realities, within which dependently originated causes give rise to dependently originated consequences according to dependently originating laws of causality. What we do and think in our own lives, then, becomes of extreme importance as it affects everything we're connected to.
The paradoxical nature of reality revealed in both the Buddhist philosophy of emptiness and modern physics represents a profound challenge to the limits of human knowledge. The essence of the problem is epistemological: How do we conceptualize and understand reality coherently? Not only have Buddhist philosophers of emptiness developed an entire understanding of the world based on the rejection of the deeply ingrained temptation to treat reality as if it were composed of intrinsically real objective entities but they have also striven to live these insights in their day-to-day lives. The Buddhist solution to this seeming epistemological contradiction involves understanding reality in terms of the theory of two truths. Physics needs to develop an epistemology that will help resolve the seemingly unbridgeable gulf between the picture of reality in classical physics and everyday experience and that in their quantum mechanics counterpart. As for what an application of the two truths in physics might look like, I simply have no idea. At its root, the philosophical problem confronting physics in the wake of quantum mechanics is whether the very notion of reality-defined in terms of essentially real constituents of matter- is tenable. What the Buddhist philosophy of emptiness can offer is a coherent model of understanding reality that is non-essentialist. Whether this could prove useful only time will tell.