The author shows that, from the kinematic point of view, the relativistic analogue of the classical divergence $q^{r}_{|_{r}}$ of a flux vector, $q^{r}$, is a space time divergence and not a spatial one. He also strives to justify the relativization of $q^{r}_{|_{r}}$ employed by Eckart in the first principle, where $q^{r}$ represents the heat flux.