Definitions are hard.
For example, this is a car.
And this is not a car.
They have a lot of similarities; they’re both mechanical devices, they’re both intended to be used as personal transport, they’re both designed to be operated on public roads, by suitably licensed people, they both operate by means of an engine of some kind, etc.
So what makes one a car and one not?
The obvious place to start is with the number of wheels, but if we definitively say, “A car has 4 wheels,” then we hit problems.
This car only has 3-wheels.
And this has only has 3 wheels and can be driven with a motorcycle licence, but is still a car.
While this also has 3 wheels, but is not a car.
Ah, perhaps its something to do with bodywork, or a roof, then!
But this car has no roof, and very little bodywork.
While this has a roof, but still isn’t a car.
And look at all the bodywork here. Not a car, though.
Something more technical, then? Something to do with the controls, perhaps?
That gets us into problems as well.
And that’s before we consider that cars can be adapted, for drivers with disabilities for example, to use hand controls, but that doesn’t stop them being cars. If you start with a car then you can chop and change all the bits you like and you’ll end up with something that is still a car…or a wreck.
Nor do this and this…
Prove that there’s a spectrum from this…
How could there be? A spectrum is a continuum, not a shit-brown blur of different factors mulched together.
It can’t be a spectrum, because there’s no way of saying which of these is most like a car.
And where would this fit on your spectrum?
Is that more car-y than a “normal” car?
As I said, definitions are hard, and if you try to isolate just one thing that distinguishes A from B then often you run into problems.
All you really need to remember is that this…
…doesn’t mean that this…
…can ignore this.
And that, at least, is easy.