It is in this way that Reid could make it one of the cardinal principles of his observations to test all that the head thinks by relating it to the rest of human nature and to allow nothing to stand, which does not survive this test. In this respect the argument he sets over against the Cartesian, 'cogito ergo sum' is characteristic: ' "I am thinking," says he, "therefore I am": and is it not as good reasoning to say, I am sleeping, therefore I am? If a body moves, it must exist, no doubt; but if it is at rest, it must exist likewise.'