 The usual way of defending it is to derive it from some general principle according to which lacking grounds for claims of a certain sort is good reason to reject those claims. The restriction of this principle to claims “of a certain sort” is crucial, since the principle that the absence of grounds for a claim is in all cases a good reason to believe that the claim is false is rather obviously false. One might, for example, lack grounds for believing that the next time one flips a coin it will come up heads, but that is not a good reason to believe that it won’t come up heads.