We could imagine a mind off in another dimension, off on the other side of the universe, at the beginning of time, at the end of time, but there is no way to conceive any space, or, distance, or barrier, in which it occupies an altogether different medium, one, closed off and separate from another, with no bridge in between.
Even if we were to consider a dream, a simulation, or even more extreme, a before life, an after life, or non existence, there is no way to conceive a mind in an altogether separate reality, completely closed off from any other, with no means of transitioning between them.
Therefore we can conclude that there is only 1 medium, all minds exist in and in relation to that medium, and we call that medium, reality.
A mind can only exist alone if no other minds exist.
That minds exist in a closed system, and can only move, and transition within it.