[UPDATE, 20 Jan 2011: I had a flash of intuition but expressed it so cryptically that now I'm trying to decrypt it for myself. I have made small revisions to the original oracle and added explanatory glosses after the three + signs.]
Because the cognitive "can't" is performatively equivalent to the metaphysical "can't", therefore determinism is false. "I can't tell you who killed your wife" because I don't know "who" he is and "I don't know who killed your wife" because "who" he is lies outside my total set of causal experience, are equivalent on determinism. Yet they are not really equivalent, therefore determinism is not a theory adequate to the real world.
Cognitive inability: "I can't answer your question because I lack the relevant knowledge/experience."
Metaphysical inability: "I can't answer your question because there is no means by which I can discover the answer." Imagine if I were asked what the happiest man in the world on 8 March 1928 ate for breakfast.
Cognitive inability: "I can't get in the room because I don't know where the key is."
Metaphysical inability: "I can't get in the room because it is bursting with flames and I am chained to the wall."
If I say, "I can't tell you who killed your wife," my inability is due to the fact that I don't know who the killer is, not to an intrinsic lack of power on my part. If I knew who the killer is, I could tell you. My response-performance would follow from my modified cognitive state. As it happens, "I don't know who killed your wife" because who he is (i.e. anything relevant to identifying him), is outside my total set of causal experience. My cognitive deficit is based on the limitations of my causal career. On determinism, my cognitive deficit is intrinsic to me, since I cannot alter my character contrary to the total causal complex which determines me. If determinism is true, nothing is intrinsically up-to-me, but only proximately inclusive-of-me in its determined ocurrence.
Yet, clearly, my cognitive deficit is not intrinsic to me, for I could perhaps acquire the knowledge I need to answer your question. This is a problem for determinism, though, since, if my cognitive (and therefore volitional) state is totally and intrinsically dependent on my total antecedent causal complex, while it is not metaphysically impossible for my cognitive state to change, it is metaphysically impossible for me to alter my cognitive state. As such, the cognitive inability manifested in my response is performatively equivalent to a metaphysical inability intrinsic to my (wholly determined) character. The cognitive "can't" is performatively equivalent to the metaphysical "can't", therefore determinism is false. An observer would have no way of differentiating a cognitive deficit from a metaphysical inability in me, since he could only go by my performative career.
Further, since determinism stipulates that there is no autonomous "self" which can function in any way independently of an agent's encompassing causal Umwelt, it follows on determinism that there is no metaphysical principle by which the agent could determine for himself how to alter his cognitive states. The alteration of his cognitive states would not be a function of his rational agency, but rather a function––or perhaps an integral?––of the causal complex which comprises both the agent and his semiotic milieu. His cognitive and metaphysical inabilities would derive from one and the same set of causal factors. Hence, on determinism, the agent's inability would be metaphysically and cognitively equivalent. Yet these modes of inability are not really equivalent, therefore determinism is not a theory of the real world. In so far as determinism confuses, or simply elides, an important distinction in reality, determinism fails as an important theory of reality.
ADDENDUM TO THE ADDENDUM:
I'm not just conjecturing about the denial of the "self" on a materialist espousal of determinism. Consider the following excerpts from "Denying the Little God of Free Will: The Next Step for Atheists? An Open Letter to the Atheist Community" by Tom Clark, Director of the Center for Naturalism.
With a little help from added emphasis, I will let the usual contradictions and fallacies speak for themselves (yuk yuk yuk).
A few points to keep in mind are that a) speaking of what determines human behavior begs the question, b) it is a false dichotomy to treat substantial rational agency as distinct from "nature" simply because it is not mechanistic, and c) in so far as the evidence for or against naturalistic claims is neither apodeictic nor deductive, such indeterminacy reinforces the need for a choice on our part for or against those claims.
With that, have a looksee:
In atheist circles it’s conventional wisdom to doubt God’s existence on empirical grounds: there’s no good evidence that such a being exists, so we don’t waste time believing in it. But there’s an equally suspect, supernatural entity that often lurks at the heart of commonsense ideas about human nature: the freely willing self.
We have, it is widely believed, the power to think, choose, and act in some crucial respect independently of those causal factors that create us as persons, and that surround us each moment of our lives. Unlike anything else in nature, human beings have a special contra-causal freedom to cause things to happen without themselves being fully caused in turn.
Sound familiar? It should, for such causally privileged freedom is a characteristic of God – the uncaused causer, the prime mover, who acts without himself being at the effect of anything. The assumption of free will, so widespread in our culture, in effect sets us up as supernatural little gods, and it’s this assumption that a thorough-going naturalism upsets. We should doubt the little god of free will on the very same grounds that atheists doubt the big god of traditional religions: there’s no evidence for it.
Just as science has radically altered our view of cosmic reality, replacing the static earth-centered heavens with the Big Bang, and supernatural human origins with Darwinian evolution, so too it replaces the soul with the fully physical person, shaped in its entirety by the complex interaction of genetics and environment. Rapidly accumulating evidence from biology, evolutionary psychology, and cognitive neuroscience suggests we are not causal exceptions to nature. There is no categorically mental agent or soul-essence floating above the brain which can exert a choice-making power that’s independent of neural processes. There’s nothing supernatural or causally privileged inside the head, just as there’s nothing supernatural outside it. …
How can we take or assign credit and blame? How do we justify punishment or praise? Without free will aren’t we just puppets, mere mechanisms playing out our fates as determined by impersonal forces?
As pressing as such questions are, they have no bearing on the truth of the matter, which, if we are naturalists, we decide on the basis of evidence, not on what we suppose must be the case. And indeed, looking at the world and ourselves dispassionately and scientifically, there really are no evidential grounds for supposing we have supernatural souls, or any categorically mental, immaterial self that makes us freely willing little gods here on earth. …
Supernatural contra-causal freedom really isn’t necessary for anything we hold near and dear, whether it’s personhood, morality, dignity, creativity, individuality, or a robust sense of human agency. We may be fully caused in our choices and behavior, but that doesn’t render us ineffective in getting what we want, nor does it upset our moral compass: we can still tell right from wrong, and we’re still fully motivated to create a world in which we flourish, not perish. As Duke University philosopher Owen Flanagan argues in his ground-breaking book The Problem of the Soul, without free will we still have self-control, self-expression, individuality, rationality, moral accountability, and political freedom (see chapter 4, “Free Will”). …
[T]he realization that we are not little gods has considerable benefits, both personal and social. First, by accepting and illuminating our complete causal connection to the world, a consistent naturalism leads to a compassionate understanding of human faults and virtues. Seeing that we aren’t the ultimate originators of ourselves or our behavior, we can’t take ultimate credit or blame for what we do. This reduces unwarranted self-righteousness, pride, shame, and guilt. And since we see others as fully caused…[,] we become less blaming, less punitive and more empathetic and understanding. …
In denying supernatural free will, naturalism focuses our attention on what actually determines human behavior. This increases our powers of self-control, and encourages science-based, effective and progressive policies….
Lastly, by showing we are fully included in nature, naturalism provides the basis for a satisfying approach to concerns about ultimate meaning and significance. Since we are not little gods, we find ourselves completely at home here on earth, part of an awe-inspiring universe, full-fledged participants in the unfolding natural order.