People die.

In cases where someone meets an unfortunate grizzly end, like being eaten, there’s an obvious reason. But more often than not, people just stop being alive.

Imagine you have no knowledge of science, how would you explain this? An hour ago, this body could move, could breathe, could do normal things. Now it can’t.

Something has changed. Something is missing. What was once a person is now a thing, a body.

It stands to reason that the missing bit is the key to what makes people human. It’s clearly not a physical thing - the body looks the same - so it must be something intangible.

Tie this to the fact that people are very good at detecting other people around them. We’re especially good at sensing when we’re being watched (in person, not through cameras, obviously). We also find ourselves in situations where we feel like we’re being watched when no one possibly can be watching.

So we have a fundamental element of human-ness as something intangible, and we also have situations where it feels like someone is there when there’s no-one around.

It doesn’t take a massive connection to associate the two.