[Reworked from comments written over at Dr. Vallicella's blog.]
Dr. V writes:
After all, if I come to see that my body, feelings, perceptions, and so on cannot be identified as my very self, then it is presumably I myself who come to this insight.
I think this assumes what you want to prove. If we look at the problem from a "top-down" perspective, where we start with what is a putatively unitary self, we begin to realize that this "self" is (1) composed of aggregates and (2) exists in relation to other phenomena-- the two ways to determine that something is "empty" in the Buddhist reckoning.
The material components of "self" are easily shown to be interdependent and not self-causing, I think. This much, at least, is common sense. The mental components of "self" exist in relation to each other, and the sense of "I" arises from their constant interaction.
This doesn't make the "I" any more real. Consciousness is always and immediately "consciousness of"-- i.e., consciousness exists in relation to that of which it is conscious. Consciousness therefore also falls into the category of empty things-- things devoid of self-nature. (Ultimately, even emptiness is empty, as Nagarjuna argued.)
The original Buddhist arguments were specifically against Hindu notions of atman-- a self that is permanent, indestructible, and unaffected by karma.
The Buddhist claim is that the "I" is an illusion. In my opinion, this claim isn't properly rebutted by the counter-claim that "I" must realize that the "I" is an illusion.
Celinda Stickles, an astute commenter on Dr. V's blog, writes:
Buddhists claim that the what's real, existing, or 'not empty' is that which is incorruptible and self-existent, a se.
But Buddhists don't claim this, because nothing is "a se" in Buddhism. From the Buddhist perspective, what's real is what's empty. Nothing exists an sich. Buddhists probably don't help themselves, however, by constantly reverting to the language of essentialism: Buddhist literature is filled with terms like "incorruptible" and "eternal" and "essential" and "fundamental" and even "Absolute" (cf. Abe Masao). Buddhists constantly refer to the "true" or "essential" nature of reality, but they aren't using these descriptors in the normally received sense.
Thich Nhat Hanh expresses the meaning of emptiness far better than I can. "The flower," he writes, "is composed entirely of non-flower elements." A flower "inter-is" with everything around it, existing interdependently, intercausally, relationally. The flower has no essence, and its existence implies the rest of the universe, just as any given number on a number line implies all the rest.
Some Buddhists use the term "Self" or even "Big I," but this is sloppily borrowed from Hinduism, where such language is far more apropos. It has to be remembered that many-- if not most-- Buddhist writers aren't interested in delineating a coherent metaphysics: their point of departure is empirical, and human experience confirms the existence of suffering. Further attentiveness reveals, for Buddhists, the fundamental nonexistence of any permanent, unchanging self.
Is Kevin at age 5 weeks the "same person" as Kevin at age 5? Or Kevin at age 50? On what grounds can we argue that the same Kevin perdures through the timestream? Surely the physical components aren't the same, nor are the emotions somehow immutable, nor are Kevin's memories unchanging (in fact, quite a few have faded!). The personal aggregates hang together closely enough to provide the illusion of a unitary self, but that self is as particulate and impermanent as a mountain or a stream or any other phenomenon we care to name. Some people find this unsettling; the Buddhist analysis of that unsettled feeling is that we suffer from attachment rooted in ignorance about the nature of things.
The Hindu argument, at least from advaita vedanta, is that Little Self is nothing more and nothing less than Big Self: atman is brahman. This evolution in Hindu thinking, largely post-Upanisadic, remakes the atman into something less vulnerable to Buddhist critique than what the Buddhists were originally attacking. The earlier notion was that the atman was like a diamond-hard core moving from body to body, unchangeable, impervious to karma. Buddhists found this to be absurd: if the atman transmigrates, then of course it's affected by karma! In Indian thinking, what the atman is depends on what it's relating to. Karma is one aspect of that relationality.*
Dr. V is correct to go right to the heart of the matter and ask the question that's plagued Buddhists from the beginning: Who attains nirvana? But it should be noted that this problem is accepted in most branches of Buddhism, especially the various Mahayana strands, as the Great Kong-an. Zen monks routinely ask their adepts: Who are you? What are you doing now? I suspect that for most Buddhists, the answer to Dr. Vallicella's question (which, in truth, isn't his question, but a question that's been asked since almost 550BCE) doesn't lie in the realm of the discursive.
My own personal preference is to avoid using "Self" (capital S) language in association with Buddhism, because it tends to confuse matters. Whether we're talking about Big Self or Little Self, no notion of selfhood is understood to be anything other than "conventional" in Buddhism.
*Western focus is primarily on objects, not relationships. If a coin is sitting in front of me, I say "The coin is in front of me." If it gets moved so that it's now behind me, I say, "The coin is now behind me." By this reckoning, the coin never changes and neither do I; it's merely our positions in relation to each other that have changed.
By Eastern reckoning, however, what the coin is and what I am are now utterly different. This way of thinking feels extremely odd to a Westerner, but it's quite common in the East.
Example: What is a table? Well, what the table is depends largely on other questions, such as where it is. If the table's in the woods, what is it? If a magician is sitting on the table, and the table's suspended in the air by a crane, what is it? Is it still a table, or is it now more of a platform? What if the table's in the woods and there are no people-- only rabbits looking for temporary shelter? The word "table" refers to what reality?
(For more on this, you'll have to speak to my Buddhism prof at Catholic University, Dr. Charles Jones. The coin example comes from him.)
Later on, Dr. V writes:
And surely nibbana must have self-nature!
I wouldn't know, but I do know that such a claim would put you against the entirety of the Mahayana tradition, which gave us the revolutionary insight that nirvana = samsara: the "real" is the phenomenal world, an insight that arose in India but received enormous support when Indian Mahayana Buddhism encountered the spontaneous, this-worldly naturalism of Chinese philosophical Taoism.
The phenomenal world has the character of emptiness (jae beop gong sang, as the Sino-Korean version of the Heart Sutra says: all-dharmas-emptiness-traits), and if nirvana is samsara, then nirvana is also empty.
This insight is one of many undergirding the belief that, while one speaks of things like "effort" and "attainment" and "buddha-hood" (or "buddha-nature"), the reality is that one is "already there," so to speak.
The paradox, then, can be phrased as another question: Why bother striving if we're already there? Again, this question, like the "Who attains nirvana?" or "Who attains enlightenment?" questions, is not unfamiliar to Buddhists.
My own feeling is that the answer to that kong-an, "Why bother?", can be found only by living life mindfully and compassionately.
Dr. V also writes:
But the failure to find x does not entail the nonexistence of x.
I see your point, but the meditator's response, with which I'm sure you're familiar because this response is pan-traditional, is that insight into one aspect of reality can provide insight into all aspects. The quest for the "real" isn't conducted by a search party combing physical territory, but by a person's contemplation of one or more specific aspects of human experience.*
True: perhaps further contemplation might reveal a deeper truth, or a truth that contradicts previously held beliefs. The Dalai Lama acknowledged this possibility when asked about the challenges of science: he said that, should science demonstrate that certain Buddhist insights were incorrect, then Buddhism would have to change. A reasonable position, I think.
*Seasoned Zen meditators will point out right away that not all meditation takes this tack. As someone who's engaged in Zen meditation, I'd agree. In Zen, the mind "has no address," as one Korean monk colorfully put it. This style of meditation is in marked contrast to both Hindu and Buddhist "one-pointed mind" forms of meditation, as well as to visualization techniques found in, say, Tibetan Vajrayana or Japanese esoteric Buddhism.