Here they are being used to test the conception of mind and matter that gives rise to them. The argument is that if mind and matter are thought of in the Cartesian way, then there would be wide-open possibilities of a bizarre kind, about which we could know nothing. So, since this is intolerable, we should rethink the conception of how things are (this is called the metaphysics). A better conception of mind and its place in nature should foreclose these possibilities. The aim is not to wallow in scepticism, but to draw back from any philosophy that opens up the sceptical possibilities.
