At like an atomic level, like really small, is it possible to determine where one thing stops and another begins? Say like where my finger stops and a key on my keyboard begins? (This might be a bad example, because a plastic key and my finger probably have quite different atoms, but still the line between them would be hard to find right.)

I'm going to say something here that is way over-simplified, but perhaps it will do.

According to quantum mechanics, of which my knowledge is very limited, such things as atoms don't have distinct boundaries in the sense you have in mind. This is because their parts (protons, neutrons, etc) don't have completely determinate positions in space (except under certain exceptional assumptions). Rather, the location of a given particle is described in terms of the probability that it is in a particular location. In fact, the same is true of macroscopic objects. Even waiving the blurriness of my boundaries, my location is not completely determinate, either.

It therefore seems reasonable to say that, no, it isn't completely determinate where your finger ends and the key begins, even if we can say which particles constitute the one and which the other (another hard problem), because it isn't determinate where those particles are.

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