A deductive approach to the relativistic law of motion presented by Kosyakov (2014 Eur. J. Phys. 35
[http://dx.doi.org/0143-0807/35/2/025012/] 025012 ) is critically analysed. We point out that the
Kosyakov argumentation is flawed and ineffective. On the other hand we show how an implicit
assumption made by the author could be used to perform a successful reasoning. Also, another method
to derive the relativistic equation of motion, in terms of Lorentz-invariant three-vectors, is
recalled.