It depends on the proposition and the system about which it is made. The reason is that the absolute "not-ness" of a condition must be tested against all possible configurations of space and time where the test condition might exist.

I can prove that my house is not painted yellow at a particular point in time, but not that it will never be. I cannot prove that I am not dirty-minded. For a large system such as a nation, a negative proposition pretty well leads to an unbounded set of test cases (infinite proving required to be conclusive). In general a negative requirement involves a huge number of possible conditions in which "the system under consideration is NOT doing X" that it becomes practically impossible. It is much, much easier to prove something conclusively if it is framed as a positive statement.



A