A discussion covers neutron (N) density and N flux; definitions of angular N density, angular N flux, angular N current, and angular interaction rate; scalar N density, scalar N flux, and scalar N current; bringing forth N transportation equation, including N source items, interaction loss items, inscattering items, space inflow and outflow, N transportation equation, and initial conditions and limiting conditions; scattering equations, including N continuity equation, simplified N transportation equation, Fick law and N scattering equation, and Fick law derivation; initial conditions and limiting conditions of N scattering theory; and general mathematical conditions of N flux.