The full, seven-dimensional neutron transport equation is developed from physical intuition, and putting that intuition into math. Aspects of neutron creation and transport are introduced as needed—neutron energy birth spectrum, flux, current, and many different types of neutron cross sections (fission, capture, scattering, total). It all comes down to balance—we equate the ways in which a control volume (like a reactor) can gain and lose neutrons of a certain energy, in a certain direction.