You may have already had experiences with invasive forces - for example, ruminating over someone that you really don't like. When you imagine a person involuntarily, you are not being physically invaded by an outside entity; and yet, your brain is physical, and you do not want to think about them - so in a way, you are. All the same, this echo of the original person is not the same entity as the actual person; they are an echo originating from that person that now exists within yourself.
Strangers, likewise, are an echo of the real world and all of the signals and abstractions in it, but they are not the real world itself.