Gilbert Ryle is famous for a catchy, quotable phrase he used to caricature Cartesian dualism: "the ghost in the machine".
The connotation of that metaphor is that if substance dualism is true, the soul is located somewhere in the body. If the soul exists, you ought to be able to find it by poking around inside the body.
My point isn't to do an exposition of Ryle's philosophy, but to use a different metaphor to illustrate substance dualism. Nowadays, not only do people have cellphones that enable to them to talk to other people, but cellphones can function as miniature TV screens which enable them to see the person they are talking to.
To take a cliche situation, suppose anthropologists discovered a tribe in the rain forest. The natives have no idea what a cellphone is or how it works. When they see faces and hear voices emanating from the cellphone, they imagine the cellphone took possession of the speaker's soul. His soul is trapped inside that little box.
But as we know, the speaker isn't actually in the device. Rather, the device is just an interactive medium to project himself.
That's analogous to the mind-body relation.