How many fingers do you have?
10, obviously, unless you've been the victim of an accident or a birth defect. Everyone knows that. You count up to ten on your fingers, for one thing.
But look at your left hand - how many fingers are on it? Little finger, ring finger, middle finger, first finger... thumb. So that's 4. But then we'd only have 8 fingers, and we all know we have 10. Unless the thumb is a finger, but is it?
Hmm. Hard to say. Wikipedia has some interesting facts about this question, and on Google if you start to type in "is the thumb", the top suggested search terms are all about this issue. It's a tricky one. People don't seem to know for sure.
But does that mean there's any real mystery about the thumb? No - we understand it as well as any other part of the body. We know all about the bones and muscles and joints and nerves of the thumb, we know how it works, what it does, even its evolutionary history (see The Panda's Thumb by Steven J Gould, still one of the greatest popular science books ever.) Science has got thumbs covered.
The mystery is in the English language, which isn't quite clear on whether the word "finger" encompasses the human thumb; for some purposes it does, i.e. we have 10 fingers, but for other purposes it probably doesn't, although even English speakers seem to be in two minds about the details (see Google, above).
Notice that although the messiness seems to focus on the thumb, the word "thumb" is perfectly clear. The ambiguity is rather in the word "finger", which can mean either any of the digits of the hand, or, the digits of the hand with three joints. Take a look at your hand again and you'll notice that your thumb lacks a joint compared to the fingers; something I must admit I'd forgotten until Wikipedia reminded me.
Yet it would be very easy to blame the thumb for the confusion. After all, the other 4 fingers are definitely fingers. The fingers are playing by the rules. Only the thumb is a troublemaker. So it comes as somewhat of a surprise to realize that it's the fingers, not the thumb, that are the problem.
So words or phrases can be ambiguous, and when they are, they can lead to confusion, but not always in the places you'd expect. Specifically, the confusion seems to occur at the borderlines, the edge cases, of the ambiguous terminology, but the ambiguity is really in the terminology itself, not the edge cases. To resolve the confusion you need to clarify the terminology, and not get bogged down in wondering whether this or that thing is or isn't covered by the term.
It's important to bear in this in mind when thinking about psychiatry, because psychiatry has an awful lot of confusion, and a lot of it can be traced back to ambiguous terms. Take, for example, the question of whether X "is a mental illness". Is addiction a mental illness, or a choice? Is mild depression a mental illness, or a normal part of life? Is PTSD a mental illness, or a normal reaction to extreme events? Is... I could go on all day.
The point is that you will never be able to answer these questions until you stop focussing on the particular case and first ask, what do I mean by mental illness? If you can come up with a single, satisfactory definition of mental illness, all the edge cases will become obvious. But at present, I don't think anyone really knows what they mean by this term. I know I don't, which is why I try to avoid using it, but often I do still use it because it seems to be the most fitting phrase.
It might seem paradoxical to use a word without really knowing what it means, but it isn't, because being able to use a word is procedural knowledge, like riding a bike.The problem is that many of our words have confusion built-in, because they're ambiguous. We can all use them, but that means we're all risking confusing each other, and ourselves. When this gets serious enough the only solution is to stop using the offending word and create new, unambiguous ones. With "finger", it's hardly a matter of life or death. With "mental illness", however, it is.