His point was that it is absurd to call the more obvious into doubt by means of the less obvious: By whatever means I might understand an argument supporting skepticism about my hands (say, the Boltzmann Brain argument), by those very means I am all the more sure that I do have hands.
Further reading: http://en.wikipedia.org/wiki/Here_is_a_hand#Logical_form http://www.overcomingbias.com/2008/01/knowing-your-ar.html http://www.gwern.net/Prediction%20markets#fn41