In my second talk in qwaq, I discussed the notion of "existence" as a relative and problematic concept. In the introduction, I gave several examples to show what questions and problems occur in this context:
a) Sqrt(-1) exists within the complex numbers, but not within the real numbers, i.e. "existence" is only meaningful here when accompanied by a "within" or "with respect to" something.
b) The fundamental physical entities have a very questionable ontological status.
c) What I call "table" is a construct of my mind. It is not identical to the physical "thing" "out there". So does that what I call table "exist"?
d) Philosophers tried to prove that God "exists". All these "proofs" seem very artificial and far away from the religious experiences that lead people to believe in Him.
e) Do "I" exist? Descarted realized that there is something that cannot be doubted: my "thinking" or "experiencing". But does this necessarily imply the concept of an "I"?
The main point of my talk was this: An ontology is not a property of reality, but a way of looking at reality. Many different ontologies can be consistent with one and the same reality.
It is, however, not completely arbitrary which one we use.
Some of them "feel" more right than others, and there are always reasons why this is so. (For Pythagoras, e.g., the existence of irrational numbers did not feel right, and therefore the diagonal of a unit square was a problem for him.) But from a LOGICAL point of view, they are all equally good.
I gave five examples of possible ontologies consistent with reality, shrinking the set of existent entities in each step:
1) The standard ontology: the external physical world exists. We are part of this world and behave according to its laws.
2) The "matrix": The world outside is virtual, an illusion common to all of us, generated by our minds. Our bodies are "avatars", used by us in order to interact with each other.
3) Solipsism: The world is a dream. My mind is real, but it is the only one. Other people do not exist. I have, however, no conscious control over the dream, and therefore I cannot just do what I want.
4) Codependent Arising: Even I am not real. There are experiencing, mental formations, sensations, feelings. Together they form the illusion of an "I" and the illusion of a "world".
5) Nothing exists. Nothing happens.
The first example "feels right" for us in most cases. But under special circumstances (e.g. meditation) we may find ourselves in a different ontology (not necessarily one of those listed above). And since there is no logically preferred ontology, there is no need to say that these "different" experiences are just "illusions".
Finally I mentioned that science is possible in ontologies 1 and 2 (referring to the list above), and that I am particularly interested how far science can be taken using ontology 2 as a starting point.
After half a year of experience with qwaq, giving a talk in this environment did not feel unusual at all. I was happy with how it went, and enjoyed the lively discussion.