It was originally proposed that (16a)-(16b) was valid if t c1 t s1 . However, although timescale separation is necessary, the validity of (16a)-( ) is actually determined by the validity of the RSA. To determine the criteria for the validity of the RSA, Segel proposed that if one assumes little change in s 1 during the approach to the slow manifold (an almost straight phase space trajectory towards the slow manifold), then it should hold that