You don't see an elephant in this picture, do you? I don't see one either. In fact, when I was there I looked around carefully, but did not see any elephants. Besides, this picture was taken in Maryland where no wild elephant has ever been recorded. And when I took this picture, there was no nearby circus or zoo reporting a missing elephant.
Therefore, I conclude that my observations and this photograph prove that there was no elephant at that point at that time.
I just proved a negative.