Just what exactly is a spirit? Nobody knows. One way of critiquing the views of theists and supernaturalists is to take a non-cognitivistic stance. Often, people find this stance to be counterintuitive or to lie beyond the purview of the standard set of arguments leveled against nonsense, but nonetheless it is a substantial one and one worth the effort of people who wish hone their reason and equip themselves with arguments against the stupidity and ignorance so ubiquitous to the human race. If you're unfamiliar with non-cognitivism, I highly recommend looking it up and finding out for yourself what it is all about.
In a nutshell, non-cognitivism refers to the position that a particular sentence, or a particular language used in a certain discipline or field of thought, does not express propositions, that is, statements that may be evaluated as true or false. To translate this into everyday terms, what this means is that if you are a non-cognitivist about something, you believe that truth and falsity do not apply to it. Consider taste preferences. Would it make any sense at all to insist that certain foods, as a matter of absolute and overarching truth, genuinely taste better than another? Can strawberry ice cream be proven to taste better than chocolate ice cream? Perhaps anchovies are the epitome of tastiness? No, obviously, this is nonsense. Food preferences are subjective; that is, they are dependent upon the individual tasting the food. Thus, a statement like, "John thinks chocolate ice cream tastes best" would be a proposition. Why? Well, this statement could be evaluated as true or false. Maybe John really likes vanilla ice cream, or maybe it really is the case that he likes chocolate. Whatever the case may be, even if we couldn't find out, it is, in principle, possible to evaluate this statement as either true or false. But consider if we simply had the statement, "Chocolate ice cream tastes best". The whole concept of "tastes best" requires that it tastes best *to some subject*; and, since it is logically possible, and in this instance actually the case, that it does taste best to some but not to others, it is quite obvious to see here that whether or not a particular food tastes best is contingent upon the preference of a particular subject.
But consider mathematical truth. Could 2+2=4 be true to one person, but not to another? No; that is absurd. Likewise, a tree is exactly as tall as it is regardless of individual beliefs, preferences, or views governing it, and the same goes for any legitimate, cognitive statement about the world, such as "France is a nation in Europe" or "Jesus was born from a virgin". It may be that two individuals differ in the information available to them or the perceptions that they have about something. From far away, a tree may look to be a certain height, and it would be true to say, "That tree looks to be 5 meters tall", while up close it could be true to say, "This tree looks to me to be 10 meters tall". Nonetheless, how tall it actually is may be some other measurement entirely - but we can be reasonably confident that trees do in fact have a particular height, independent of individual perspectives. Reality is not subjective.
So how does this apply to spirits? Do which category do statements about spirits fall into? Well, consider how it is we come to present a statement as a proposition: it is related to how we come to know things and how we justify that knowledge. There is really only one process for justifying a position - reason. That's not to say we must come to know all things in the same way. Some knowledge can be ascertained deductively - mathematics, definitional truths, and so on. Such knowledge is derived from, in a sense, pure reason - we do not need to appeal to evidence to prove that 2+2=4, but to definitions of these terms from which their truth or falsity follows necessarily. The second method is by induction. This knowledge does not have the ring of necessity to it. Inductive knowledge is knowledge that depends upon our acquisition of information from the outside world. When we draw an inductive conclusion, it must bear some relationship to those proposed facts, which are intended to support it. The way we come to know how to treat cancer, or that gasoline is flammable, or generally any scientifically evaluable knowledge about the world, is derived through induction.
It is rather obvious that the existence, or the nature, of spirits cannot be demonstrated by an appeal to pure logic. One cannot conjure into existence beings on the basis of argument independent of an appeal to the outside world. If they can, I certainly haven't seen any compelling arguments, and, if you think there are some, I'd be interested in knowing about them. Leaving the rather dubious possibilities of presenting a logical argument for the existence or nature of spirits aside, that leaves only with inductive arguments for spirits - arguments that would incline us to think spirits probably exist or that we can have knowledge about them that depends upon our knowledge of the world around us. This could come in the form of hard physical evidence, like the remains of spirits; in the form of accounts or witness reports of spirits, it might even come in the form of indirect evidence - signs left that indicate the presence of spirits, or something to that effect. Essentially, if we are to believe that spirits exist and that there is something we can know about them, we must present REASONS to believe this. In the absence of reasons to support a belief, a belief is, by its very nature, unreasonable, which is just another way of saying it's irrational or absurd to believe whatever it is that's believed.
So what evidence, what knowledge do we possess about the nature of or the existence of spirits? I assert, unequivocally, that there is none. We know nothing about what a spirit is, and we have no credible evidence whatsoever that such things exist. Even if someone shows us a grainy video of some moving light in the darkness, why should we presume spirits give off light? Do they or don't they? Often, we are told, a spirit is an immaterial thing. But does this tell us anything about the nature of a spirit? No, it does not. Telling us something isn't material doesn't tell us what it is, it tells us what it isn't. If I told you I had an object, and that it was not round, does this inform you of what shape it actually is? No. That's not to say such information is meaningless, but there's a problem with my assertion that it is not round. In order to know what something isn't, one must know something about what it is. How can I know my object is not round unless I know something else, something positive, about its properties? The simple answer is that you can't. Thus, when we purport to offer knowledge of what spirits are not, we aren't even justified in that. Just how is it we know spirits aren't material? And if they're not, what are they? Nobody knows.
I assert that it's not simply the case that there isn't any evidence that spirits exist or not, but that people who speak of spirits, ghosts, and the like, literally have no idea what they are talking about. They are uttering incoherent gibberish. Without some coherent, definable characteristics which lend some content to the term "spirit", the meaning of the word is as ethereal as spirits are purported to be. The next time someone claims that a ghost or a spirit exists, before demanding evidence of their claim, demand that they explain what a spirit or ghost is, and how they know this. I, for one, doubt they would provide a satisfactory answer, and I won't hold my breath waiting for one.