Sidis pointed out that a general characteristic among physical laws is the irreversibility in time. To test reversibility, he created the concept of reverse universe. In this universe, the three dimensions of space are unchanged. The only difference is the direction of time. As he wanted to test any given physical law, he mentioned that all positions in space remain absolutely the same in the reverse universe as in the real one. This implied that the path of a moving body will remain the same. But, of course, the body would move in the reverse direction. It follows that all the velocities must remain the same and it is the same for its acceleration. And because the mass is not related to time, it also remains the same. It is exactly the same for energy. Another point is to consider that all logical relations of facts remain the same. Sidis remarked that even if the reverse universe destroys all mental phenomena, logical relations remains. But we have to translate cause in effect and effect in cause. And this is the difference which makes the difference.