Given this starting point, the various assumptions underlying the Boltzmann equation carry over to Chapman–Enskog theory as well. The most basic of these requires a separation of scale between the collision duration τ τ c and the mean free time between collisions τ τ f : τ τ c ≪ ≪ τ τ f . This condition ensures that collisions are well-defined events in space and time, and holds if the dimensionless parameter γ γ ≡ ≡ r c 3 n is small, where r c is the range of interparticle interactions and n is the number density. In addition to this assumption, Chapman–Enskog theory also requires that τ τ f is much smaller than any extrinsic timescales τ τ ext . These are the timescales associated with the terms on the left hand side of the Boltzmann equation, which describe variations of the gas state over macroscopic lengths. Typically, their values are determined by initial/boundary conditions and/or external fields. This separation of scales implies that the collisional term on the right hand side of the Boltzmann equation is much smaller than the streaming terms on the left hand side. Thus, an approximate solution can be found from