肇論新疏 3.252

3.252

初四句，難三乘有差，以三一互違，故非究竟，則無常也。次二句，躡前以難三位之殊。升降，高下也。中謂折中，亦正也。

The first four clauses challenge the differences among the three vehicles; because three and one contradict each other, it is not ultimate, and hence impermanent. The next two clauses follow up from the previous to challenge the differences among the three stages. "Ascent and descent" means high and low. "Mean" means compromise, and also what is correct.