Here's a summary of an old debate in philosophy between Saint Anselm and a little-known monk named Gaunilo, which dates back to the Eleventh Century Anno Domini.
Anselm: God is that which no greater can be conceived.
Gaunilo: Meaning what, exactly?
Anselm: God is perfect.
Gaunilo: So I cannot conceive of anything greater than God?
Anselm: Correct. Otherwise He would not be perfect.
Gaunilo: So if God is perfect, God exists, right?
Anselm: Correct. Otherwise He would not be perfect.
Anselm: Because perfection is greater than imperfection.
Gaunilo: So perfection is necessarily existent, while imperfection is not necessarily existent.
Anselm: Yes because if something existed it would be greater than if it did not exist.
Gaunilo: So because God is perfect by definition, by definition he must also exist?
Gaunilo: Aren't you defining God into existence?
Gaunilo: But I can define things into existence, using that method.
Anselm: How so?
Gaunilo: Imagine a tropical island which no greater can be conceived.
Anselm: Okay, a perfect tropical island.
Gaunilo: The perfect topical island necessarily exists, but the imperfect ones may not necessarily exist, because an existing tropical island is always greater than a non-existing tropical island.
Anselm: But that's silly, where's the perfect tropical island supposed to be?
Gaunilo: My point exactly - I built its existence into the definition.
Anselm: Why should I accept your definition?
Gaunilo: Why should I accept yours?
Anselm: I'm not going to accept a fool's argument. Only a fool denies what he knows in his heart to be true.
The question: What's the difference between a perfect tropical island that exists and a perfect tropical island that doesn't exist? is relevant to more applications than theology. Today we have the debate about qualia and physics, which is very similar. The only difference between the islands is, in the end, the existence of the islands themselves.
Kant re-articulated Gaunilo's point in The Critique of Pure Reason when he wrote that existence was the realm in which we call object into question, it's not part of the question itself. The possible world calculus cannot help us with actual and non-actual if we say something is greater if it is in existence versus non-existence.
Think about the way qualia is debated, analogously.
We can ask: What is the difference between knowing all the truths of physics and perceiving all the truths of physics?
The difference, in the end, is rather trivial. (In fact the difference is the différance.)
If we know all the truths, we know what perceiving them is going to be like as well. It depends on how pervasive we take knowledge to be. We have to think about it this way because knowledge of truth in this debate is reduced to an anthropomorphized version of empiricism. When we're talking about knowledge of the truths of physics, it should be in the pervasive sense. It's not empirical knowledge, as physics is normally taken to knowledge about, it's the what-it's-really-like in the ontological sense kind of knowledge.
If there is a difference between knowledge of physics and the perception of physics, then physicalism is false. If there is something peculiar about the way we perceive the truths of physics that is different from physics itself, then the world is made of more substance than physics alone. Which means the ontology of physics cannot explain everything.
That is why the qualia debate is very important. It is not a debate about the mind alone. Contemporary philosophy has merely reduced it to a question about that for simplicity. It means that, if there is more to ontology than physics, physics could never be complete as an explanation.
How is this debate like the debate between Gaunilo and Anselm? I have come up with a rather short list (but still growing! submit more if you'd like).
The easy similarities:
- Both entail an ontology which is not completed by physics alone.
- Both are ontological arguments for the existence of a particular thing.
- Both are a priori arguments for the existence of a particular thing.
- Both God and qualia are defined into existence through analytic terms.
- Only the fool "denies what he knows to be true in his heart" upon introspection, in both cases.
- Each defines the object by referring to a transcendental signifier, something that has no reference in the structure of language, which can be defined by other reference points.
- 'Absent qualia' and 'absent gods', if you will, are said to exist in some possible worlds, but they are seen as imperfect if absent. Hence we assume there are worlds where they are present.
- It is argued that knowledge of physics will not obtain knowledge of qualia nor knowledge of God.
- If it is possible that worlds exist in which absent qualia and absent gods are truths, physicalism is said to be false.
- If qualia or gods are possible, physics is not complete.
Some might point out that contemporary physicalists accept the notion of qualia, while maintaining that physics is complete. This is the same mistake a soft-Gaunilo might make with with respect to God. Allowing that God is possible means that physics is an incomplete explanation for all possible worlds. If the analoticity of our language transcends signification, we ought not allow them possibility. It would be like allowing the perfect tropical island possibility.
Contemporary physicalists who accept qualia argue that qualia can be reduced to physics. They argue that when speak about "qualia" we are signifying something that is actually physical. However, they have allowed that there are explanatory gaps in physics, and introduced all sorts of problems using reductive methods to explain why qualia are ultimately physical. No one argues, on the other hand, that God can be reduced to physics. Or that when theologians say "God" they are really referring to something physical.
Another school of thought, known as eliminative materialism, argues that the sign 'qualia' signifies nothing at all. Instead of reducing something that does not exist to facts about the world, eliminativists eliminate the signifier altogether. Why allow that the sign signifies anything at all?
A contemporary philosopher at the University of Notre Dame, Alvin Plantinga, has since taken up the old debate in Anselms's defense. He argues that Kant and Gaunilo were wrong, and that if it is possible that God exists, then physicalism is false, and it is therefore rational to believe in God's existence. For the dualist, physicalism as a possibly false explanation is weighed against immaterialism as a possibly true explanation, and in every debate there are those who unknowingly allow the dualist more possibilities than is necessary to give.
Gaunilo, by the way, was not burned at the stake for replying to Anselm's treatise. And eliminative materialists are not denied tenure for their own treatises either.