A surprisingly nasty problem in philosophy is trying to determine what existence is. Or, perhaps, what I should ask is what does it mean if something exists?
Lots of answers have been tried, such as being extended in space. This means that anything that somehow takes up space exists. Lots of philosophers argue that we can never know what actually exists, because all that exists to us is what's in our own mind.
That's a pretty good argument, I think. Even modern psychology has, under one of its assumptions, a recognition that the world we see is not how the world is. The object we "see" is disconnected from "us" by the object, the light that travels to our eye, our eye receiving that light, the signal of the eye coming to our brain, and our mind eventually paying attention to that signal.
There are FOUR middlemen between the object and us. As far as psychology is concerned, the world appears to us not as the world is, but in such a way as to aid us in making decisions. That's called evolving to correctly interact with an environment. Red is red because it was beneficial to see red as something bright and outstanding. Be it for fruit or blood.
So does anything "exist" if all we can know are ideas in our own heads? While I think this reinforces my support of coherentism, I also think that things actually exist, whatever that means. I think existence might be a primary concept. In the same way that some words are basic words that cannot be defined beyond being a basic concept. Saying that something exists means it exists. It's something of a tautology, I know, but I think it's as far as we can go. Anything beyond that is impossible to define and, importantly, useless. I'll explain.
(UPDATE: I've changed my view on this. I entertained the possibility of existence being a primary concept and have rejected it entirely. All examples I can conjure require sense perception at some point for it to be shown, a hypothetical entity without any sense wouldn't have a concept for existence since the only thing would be what's called the metaphysical agent, i.e. the person or "I." As far as I is concerned, I doesn't exist, I simply is an assumption in the same why that the eye doesn't exist as far as the eye is concerned, it only assumes its existence to explain why it's seeing. Existence is not primary, it must be linguistic.)
If we were to dig further into the issue, I'd have to say that existence is predicated on sense. If we sense something, it exists. I know dogs exist because I've seen them. I know small things exist, because through experiments, things like atoms have been confirmed. As such, something can only be said to exist if it has been detected.
An issue I see is with a metaphysical proposition. "There exists a planet with two-headed people on it" is either true or false. This is a metaphysical issue because it is attempting to reach beyond our sense to a greater form of truth.
It is definitely true that a planet with two headed people either exists or doesn't exist. But until we detect that planet, can we actually say that the planet exists? Our language is limited by our being human.
If we detect the planet, we can then say that it exists and that it existed in the past, but it is a post-hoc philosophical statement. The planet did not exist before we detected it since the very word 'exist'
relies on our human condition and what words mean to us.
Existence as a potentiality would fit this problem. Anything that has the potential to be detected exists, as such, the two-headed people planet exists because we could detect it if we were there, but we can't reach past our own sense. The metaphysical issue of the planet's existence is resolved, but it is a solution that is beyond us. We cannot say that the planet exists until we've seen it, even though it definitely exists, whatever that means.
I think in general usage, people will use the word 'exist' in a nebulous way. A good definition for 'exist' in popular use would be that something is "out there." How is something "out there?" Not sure, really. Since trying to explain it further runs into metaphysical issues that are beyond human sense and logic, we can only say that something exists if it exists. How do we know something exists? By sensing it.
So yes, there exists things that we have not sensed, but it still seems to make sense that they exist. It makes little sense to try and say anything beyond that because we are already at the limits of human language, logic, and sense.
I'm a stickler for well-formed logic and meaning, so I'm inclined to say that existence is entirely dependent on sense. Dogs exist because I've seen them. Atoms exist because I've seen the results of particle accelerators. The two-headed people planet does not exist, even if it is somehow "out there" doesn't matter. My word "exist" relies on the human experience and as such it only makes sense if its definition comes back to it.