In an argument over the existence of something, it is a prerequisite that that “thing” be defined precisely in terms of its properties – the properties that we assume the “thing” possesses by definition. If I claim the existence of a “thing”, you have no reason to believe or disbelieve me unless I clearly specify the meaning of the “thing” in terms of its properties.
It is only when the description of the “thing” is clear and unambiguous to you and me that we can argue over the existence/non-existence of the “thing”. There is simply no question of the existence/non-existence of a “thing” unless we have a common definition of that “thing”.
Take, for instance, the question of God. So it is like this: say, you claim the existence of God. Your friend claims the non-existence of God. Yet another friend claims, “I don’t know!”, and refuses to take any stand. The third friend has taken the safest and technically the best ‘stand’ (which is, NOT to take a stand on the existence/non-existence of God) in any circumstance. It may be his motivation is his fear of being wrong if he takes a stand and facts contradict it. It may be he is not bothered about the question. It may be he hasn’t been able to form a clear opinion despite having pondered over the question. Or it may even be (as in my case) he iss not clear on what the other two friends mean by ‘God’. The point is: any claim as to the existence or non-existence of a “thing” should necessarily be accompanied by a definition of the “thing”, as the claimer perceives it, in unambiguous terms. And in order to have a genuine discussion/debate on the existence/non-existence of the “thing”, all three friends must agree on a common definition of the “thing” whose existence is in question. Otherwise each may use his own definition of the “thing” and each of them may be correct, yet an endless argument will ensue because of the simple reason that they did not take enough care to be clear on what they are talking about and what they are NOT talking about. The same applies to discussions not only about the existence/non-existence of God but the existence/non-existence of any “thing” we can conceive of (including, of course, the very exciting concept of aliens or intelligent life).
Concepts exist in the mind. A concept is basically a broad framework that seeks to describe within that framework some possible entity in the real world. Whether that entity exists in the real world is another matter altogether. It is something to be ascertained through experiments. So, when we argue over the existence of some “thing” we are talking about its existence in the real world. For in the mind a thing exists the moment you conceive of it. And it exists for the one who conceives it, not for someone who hasn’t yet conceived it. I’m not talking about that kind of metaphysical existence. So, yes, even I have to be clear on what I mean by existence in the context of these reflections. That the concept of God exists in my mind doesn’t mean I can say for sure that there’s a corresponding ‘real’ God out there (or, assuming omnipresence, everywhere). Nor can I say that there is no real God anywhere. I can only point out facts and reasons that correspond to my conception of the idea of God. There may be facts that correspond to someone else’s idea of God in some other respects which I haven’t considered. So, essentially, it turns out that my idea of God is as incomplete as the others’ idea of God. We have to work on putting our ideas together in a consistent fashion. Note that whatever I’m saying with regard to the concept of God applies equally well to any other concept that anyone can conceive of. Why I often find ‘God’ a useful concept to illustrate the idea I want to convey is because this concept intrigues quite a number of people, it being quite intrinsic to the social psyche.
The other thing is about the importance of evidence – not only in favour of existence of a thing (in case the “thing” exists) but also in favour of the non-existence of the thing (in case the thing doesn’t exist). People say they can’t believe some “thing” for want of evidence. But they forget that they can’t either disbelieve that “thing” for want, of course, of evidence. So the only alternative in case of complete lack of evidence in support or against the existence of a thing is to refuse taking a stand on its existence/non-existence. For you don’t have sufficienct information. Or maybe you don’t have the time or the energy or the interest to process the available information in making a comprehensive judgement. Carl Sagan the astronomer said it best:
“Absence of evidence is not evidence of absence.”
Hence, be careful the next time you claim something doesn’t exist. That which does not exist is the entity that corresponds to the concept that you conceive in your mind. It isn’t the concept that doesn’t exist, it is the corresponding real entity that actually doesn’t exist. The concept necessarily exists for you to be able to form a judgement on whether the corresponding reality exists or not.