Physically measurable reaction rates are proportional to | M | 2 , thus so far nothing is different. However, consider that there are two different routes : a ⟶ ⟶ 1 b and a ⟶ ⟶ 2 b or equivalently, two unrelated intermediate states: a → → 1 → → b and a → → 2 → → b . Now we have: