It is observed that the formal mathematical adjoint of the integral form of the solution of the Boltzmann equation is not the same as, and is not a solution for, the formal mathematical adjoint to the integro-differential form. If the concept of importance is to have a unique physical meaning, there must be a basic physical difference between the adjoint integral and the integral solution to the integro-differential equation. We show that such a physical difference can be specified, that the concept of “inverse causality” is unnecessary, and that normal “forward” causality is sufficient to derive the importance from first principles. The resulting equations for the importance distributions are then shown to be completely consistent with all requirements of orthogonality between these distributions and the neutron distributions.