The straightforward application of energy and linear momentum conservation to the absorption/emission of photons by atoms allows to establish the essential features of laser cooling of two levels atoms at low laser intensities. The lowest attainable average kinetic energy of the atoms depends on the ratio \Gamma/E_R between the natural linewidth and the recoil energy and tends to E_R as \Gamma tends to zero. This treatment, as the quantum mechanical ones, is valid for any value of the ratio \Gamma/E_R and contains the semiclassical theory of laser cooling as the limiting case in which E_R\ll \Gamma.