  The logical inference approach to quantum theory, proposed earlier [Ann.
Phys. 347 (2014) 45-73], is considered in a relativistic setting. It is shown
that the Klein-Gordon equation for a massive, charged, and spinless particle
derives from the combination of the requirements that the space-time data
collected by probing the particle is obtained from the most robust experiment
and that on average, the classical relativistic equation of motion of a
particle holds.
