the electric and magnetic fields and this leads to the conclusion that $\mathbf{F}=q( \mathbf{E}+\mathbf{v}\times \mathbf{B})$ is the Lorentz force. We should emphasize that this procedure to obtain the Lorentz force was roughly sketched out by Feynman in his handwritten notes. In our opinion, however, Feynman’s route to the Lorentz force is criticisable: The hypothesis of a force linear in the velocity is not sufficiently well justified. But we must also recognize that the derivation of a Lorentz-like force from relativistic considerations and the assumption of a force depending linearly on velocity are conceptually interesting.

-   De Luca et al. [@3] assume the relativistic action $S=\int_{t_1}^{t_2}[-m_0ds-qA_\mu dx^\mu]$, where $A_\mu$ is the 4-potential (they now use relativistic notation). They then vary this action to find the force $\mathbf{F}= q[-\nabla\Phi-\partial\mathbf{ A}/\partial t+\mathbf{v}\times (\nabla \times \mathbf{A})]$. Comparison of this force with the previously obtained Lorentz force yield the relations $\mathbf{E}=-\nabla\Phi-\partial \mathbf{A}/\partial t$ and $\mathbf{B}=\nabla\times \mathbf{A}$ which imply the homogeneous Maxwell’s equations $\nabla\cdot \textbf{B}=0$ and $\nabla \times \mathbf{E}=-
       \,\partial \mathbf{B}/\partial t$. This procedure based on the least action principle, which starts with potentials and ends with the homogeneous Maxwell equations, was not drawn in Feynman’s handwritten notes. In getting the homogeneous Maxwell equations, De Luca 