My Trusty Water Bottle exists as a concept as well as it does as a physical object. It's the water bottle that I currently have and I will always refer to it as the same thing, even though by this point there have been almost a hundred individual bottles. Is it really the same?
When my friend Matt said, in 2008 "Why the flonk do you always have that water bottle with you these days?" (except with a slightly ruder word than 'flonk') was he talking about the same thing as my friend Tülin when she said last year "I can always recognise you by your water bottle!"? They were both talking about my Trusty Water Bottle after all.
"But there are actually lots of Trusty Water Bottles!" do I hear you say? Well, no, there aren't. The one in my pocket is my Trusty Water Bottle, not the one that was melted down in a recycling centre. There's always only one Trusty Water Bottle!
"But they are physically different objects!" and? So am I, really. Every atom in my body at this moment will have been replaced since 2008. But I'm still the same person, aren't I? Just like my Trusty Water Bottle. So, if there's been multiple Trusty Water Bottles, there must also have been multiple Adam Randalls.
But then, now that I think about it, how would I even know? I certainly like to think that I have changed since 2008, so how can I be one hundred percent sure I am the same person? What if I went to bed one night, my mind dissolved in my sleep and was replaced with a new one with all the same memories as the one it replaced. It'd be impossible to know.
My Trusty Water Bottle may have actually been multiple bottles, but that doesn't change the fact that there is one Trusty Water Bottle, much like the ever changing atoms that make up my body don't change the fact that there is only one of me! Perhaps we put too much emphasis on the physical side of things, when, really, everyone and everything are just ideas, in the end.
(Or, basically, The Ship of Theseus)