> There is an easier approach. "all ravens are black" is logically equivalent to "all non-black things are not ravens". Proof of one proves the other. So why no

> There is an easier approach. "all ravens are black" is logically equivalent to "all non-black things are not ravens". Proof of one proves the other. So why not test the second, equivalent formulation instead? This equivalence only holds in a logic in which the law of excluded middle is assumed. For example in intuitionistic logic this is not true.