Ontology
"That department of the science of metaphysics which investigates and explains the nature and essential properties and relations of all beings, as such, or the principles and causes of being".
There is an alternate definition which applies in the field of Artificial Intelligence which is a sort of formal definition of the relationships and concepts in a particular field or "domain", but I'm not going to consider that definition.
We can point to anything and say "That exists!" This is true(1) even if the "thing" we point at is an illusion or chimera. Even if we were strapped down, unconscious, with wires feeding impressions into our brains, those impressions would exist, would they not?
But when we consider a table, a 'real, physical object', isn't that different from, say, a unicorn, or a triangular square, or truth? Hmm, both 'real, physical objects' and 'imaginary objects' exist as mental states of the brain. I don't believe that there are any criteria by which we can reliably mentally distinguish between 'real' and 'imaginary' objects. That doesn't mean that there is no distinction of course.
I had a discussion with someone as to whether or not you could conceive of something without conceiving of its surroundings. My feeling is that a conceptual unicorn cannot be considered without its surroundings - a field, or a forest. The other person did not agree. My feeling is that this is an effect of concentration - if you look at a real object, its surroundings become less prominent to your awareness, but they don't completely disappear, and the same, I believe holds true for mental objects. If you mentally draw back from a mental object it is not (unless you choose to imagine it so) suspended in a void.
It is tempting to consider two levels of 'existence', mental and physical. A physically existent tree can be seen to be existent by more than one person - there appears to be an objective component, while the unicorn can be communicated but each person has their own mental object of a unicorn which is distinct from that of the other person.
But I don't like that idea. The easy way out is to doubt the physical realm - it is all in our heads. But the mere concepts of "our" and "heads" seem to indicate otherwise, both intimating something outside of the mental realm. Solipsism doesn't ultimately appeal, even though it is simple and requires nothing outside of the mind - economically in the extreme!
One solution, and I'm not sure if I like it, is to postulate an 'imaginary' realm containing things like unicorns and four-sided triangles. This would then bear the same relation to the mental world as the real world does - 'real' and 'imaginary' both somehow feed into 'mental' realm.
I'm not sure what I think the answer is. I'm not sure I've even got near the topic of ontology. I need to do some more reading!
(1) "What is truth?" cries Pilate as he washes his hands. Pilate asks the philosophical question of Jesus, but although many have tried, before and after Pilate, none have really answered the question to everyone's satisfaction.
