  An elementary mathematical example proves, thanks to the Routh-Hurwitz
criterion, a result that is intriguing with respect to today's practical
understanding of model-free control, i.e., an "intelligent" proportional
controller (iP) may turn to be more difficult to tune than an intelligent
proportional-derivative one (iPD). The vast superiority of iPDs when compared
to classic PIDs is shown via computer simulations. The introduction as well as
the conclusion analyse model-free control in the light of recent advances.
