True, we can not see all. Another critique of Clifford, might be that evidence itself can't see all. In the example of the ship-owner, he could've investigated for problems and deemed the ship's condition permissible, and still a certain storm or wave may have destroyed it. Still though, a part of me thinks Clifford has a good argument (the need to be critical of our beliefs), but not to the point of extending blame to those who he feels having been critical long enough or to the extent that he chooses.