Thus, the distance from the critical manifold to the slow manifold is asymptotically negligible as → 0. The initial velocities of the indicator reaction ( ṡ2 and ċ2 ) are identically zero, and experimental initial conditions start on the critical manifold. Consequently, we will we not attempt to define a fast timescale that is analogous to t c1 . Instead, our strategy will be to estimate a suitable depletion timescale for the indicator reaction, and designate this timescale as either "fast" or "slow" depending on how it scales with respect to the depletion timescale of the non-observable reaction (i.e., t s1 ).