What is it for something "to exist"?
It's pretty straightforward from how we talk about it all the time, but the implications are quickly overlooked.
In order for something to exist it must be sensible; that is you must be able to see, touch, taste, smell or hear it. Those are the only ways in which things have the ability to exist; things exist and not everything we talk about exists.
Nouns do not always name people, places or things. All people, places or things exist(ed).
A unicorn is not a thing, Harry Potter is not a person and heaven is not a place.
If a unicorn were a thing we could go and see it like I can go see a tree. I would be able to point ot it, tell you what color it is and how fast it can run. Harry Potter is not someone I can meet, talk to or hang out with: he is not a person. Heaven is not a place, because if it was we could go there, you could give me directions to it and show me where it is on a map.
Imagine someone telling you they want to show you something but you can't see it or touch it, smell it, hear it or taste it. They would be showing you exactly that: no-thing.
Understanding the proper use of words led to some interesting (I thought) points about what does exist and what does not.
Feelings, dreams and pain do not exist: they occur.
People often talk about "having" pain, but if you really had pain you could show me your pain (not where it is). "Having" something denotes being able to give it away; can you give me your pain? No. Where is your pain? You point to where the pain is occuring, but you are at a loss to show me the pain itself. If I cut you open can I see the pain? Clearly not. Pain does not exist: you can't touch it, see it (you see the manifestation of pain in a person's behavior), or in any other way sense it.
Feelings and dreams follow along the same path. I do not "have" feelings, feelings occur and I manifest them in behavior. When I'm sad, crying is the manifestation of that sadness. The sadness is not in me somewhere, you can't cut me open and find happiness, sadness, madness and the like. Your dreams are not had, you can't give me them. They're also not anywhere "in you".
The big whamoo coming is that there is no such thing as a "mind". At least not in the sense most people consider it to be: some sort of container that holds thoughts, dreams and things like that. There is no mind inside you, you won't find it when I cut your brain open. The content of a person's mind is identical to their verbal and phsyical behavior.
You don't have something "on your mind" like you do "on your arm". If this were correct it would be proper to ask "Where do you have something on your mind?" which clearly is not.