  And, when Descartes was pressed by Elizabeth as to how mind and body  interact,[4]  she rightly regarded his answers as unsatisfactory. The basic problem here is set by dualism: how can one substance causally affect another substance of a fundamentally different kind?  In its Cartesian form, the difficulty is particularly severe: how can an extended thing, which affects other extended things only by mechanical impact, affect a mind, which is non-extended and non-spatial?