  The true source of the reversibility problem was only identified by Burbury (1894a) and Bryan1 (1894), by pointing out that already the Stoßzahlansatz contained a time-asymmetric assumption. Indeed, if we replace the SZA by the assumption that the number of collisions is proportional to the product \(f(\vec{v}'_1) f(\vec{v}'_2)\) for the velocities \(\vec{v}'_1, \vec{v}'_2\) after the collision, we would obtain, by a similar reasoning, \(dH/dt \le 0\).  The question is now, of course, we would prefer one assumption above the other, without falling into some kind of double standards. One thing is certain, and that is that any such preference cannot be obtained from mechanics and probability theory alone.