I think it's really hard to define this fully. The most relevant parts are probably the Pauli exclusion principle (which prevents two fermions from occupying th

I think it's really hard to define this fully. The most relevant parts are probably the Pauli exclusion principle (which prevents two fermions from occupying the same point in space if they are otherwise identical), and the size of elementary particles (which, as far as we can tell, is 0). Overall this means that you can get two fermions arbitrarily close, but not to overlapping positions. Since they have no know dimension, they can't "touch" unless they are completely overlapping, and since they can't overlap, and since the electromagnetic repulsion increases rapidly as you get closer and closer together, I think the parent poster is right - regardless of inertia, they will always have some kind of electromagnetic interaction that will change their course or nature before actually "touching". To add a bit of a complication, if two particles that have mass, even one as tiny as the electron, come closer together than some minimum, general relativity predicts that they would collapse into a (really really tiny) black hole. Of course, we don't know how and if general relativity really works at this level, so the relevance of this is unclear.