Something doesn't have to have a definite boundary to exist and I don't think it really matters to the author's point. This may or may not have been intentiona

Something doesn't have to have a definite boundary to exist and I don't think it really matters to the author's point. This may or may not have been intentional by the author but you could also interpret the sentence as referring to whatever it is that does exist that we label neutrons as opposed to the models we currently use to define that thing.