Philosophers are often asked whether they can provide a defence against hostile theories which are said to be “undermining the foundations of morality,” and they often try to do so. But before anything of this kind is attempted we should surely ask whether morality could be threatened in this way. If what people have in mind is simply that the spread of certain doctrines leads to the growth of indifference about right and wrong there is no philosophical problem involved. So long as we treat the matter as a case of cause and effect it will belong rather to the psychologist than the philosopher, and we have no reason for questioning that correlations of this kind may exist. But this is not the assumption, or not the only one, for people undoubtedly do think that if certain doctrines could be proved then moral judgment would have been shown to be “nonsensical,” “meaningless,” or “invalid,” so that thereafter it would be not merely difficult but positively irrational to formulate and attempt to follow moral principles. It would be simple enough if the attack was supposed to be against some particular moral code, for there are recognized ways of arguing that a thing is not right but wrong. But when it is morality in general which is to be disproved or discredited it is difficult to see what this means or how it could be done. What would have to be shown is not that this or that is not right, but that nothing is—or not in the old sense so that attacking moral judgment is not like attacking a theory but more like attacking theorizing itself, which shows where the difficulty lies. If something is stated it can be denied or disproved, but a moral judgment does not contain statements except about what in particular is right or wrong. Yet many people, though they would probably reject a request for a justification of morality in the form of some argument as to why we should do our duty, feel that morality would be in a positive sense unjustifiable if certain supporting truths were knocked away from the structure. This may indeed be so, but we are unable to show that it is, or to explain the matter by appealing to “presuppositions” of morality, which besides being far too vague would too easily include much that was linked merely psychologically to the recognition of obligation. I propose, therefore, to look at some specific arguments which are supposed by those who resist them to constitute a threat to morality, and to ask whether this supposition is justified.