First, we recognize that we are committed, somehow, to the notion of the P-world: the domain of first person singular, the now, extended into the specious present.

Then we recognize that it is hard to find the borders of the P-world in space-time.  We can track the limits of the senses, but memory, feeling and emotion ensure that we have a hard time finding borders.

Then we realize that the specious present extends into the future through the (Bayesian) notion of prediction.  Most things in the P-world are utterly predictable if we adopt a sufficiently modest temporal window.

Agents, or other sources of action, in the P-world introduce limits to predictability.  They thus represent a border.

You are the border of me.

Addendum: If we engage in strictly synchronized behavior, then we become much more predictable for each other, and the border recedes.

PPS: The hell of solitary confinement in prison is the openness, not the fictitious boundaries.