It ' s the difference between a valid argument and a sound argument . All sound arguments are valid , but not all valid arguments are sound . Consider the following valid argument . 1 . If I have a purple hat , then all marshmellows are poisonous to humans . 2 . I have a purple hat . Conclusion All marshmellows are poisonous to humans . This is valid , because if all the premises are true , the conclusion must be true . However , as it happens neither of the premises are true . Which means that while valid , this argument is not sound . Consider the following that is both valid and sound 1 . All mammals must breath air . 2 . Dolphins are mammals . Conclusion Dolphins must breath air . Many religious arguments are of the first type . They postulate premises that , if true , would require the conclusion to be true . However their arguments are not sound because the premises are not true .