This paper is an addendum to the article “On the Evolution of Approaches to the Space- Time Symmetry”, Natural Science, Vol. 10, No. 3, pp. 81-84, 2018. The analysis is based on the Galileo’s postulate that all inertial reference frames are equal. The postulate results immediately in the absolute speed limit, which is identical for all bodies in all reference frames. Two kinds of elementary particles are admitted: unhurried and restless ones (ex-emplified, accordingly, with electrons and photons). Any particle may be treated as wave packet, the particle mass and momentum depending on the reference frame. Thus, space-time-symmetry is the natural background for the relativistic and quantum theories.