The Cartesian 'other' is the counterpart to the Cartesian Self. According to Descartes, there is a divide intrinsic to human consciousness, such that you cannot ever bridge the space between your own consciousness and that of another.

This 'other' is in essence theoretical, since you cannot ever be empirically shown such an 'other'.

Put differently, Descartes concluded famously Cogito Ergo Sum, "I think, therefor I am", but realized that (according to his Wax hypothetical, as discussed on the wiki page for Descartes) you could never similarly demonstrate the existence of the 'other'.