Now we proceed to more specific dynamical concepts. We begin with the concept
of "mass". In order to introduce correctly the __new__
physical concept of the
"mass of a moving body" into SRT, it is necessary, first, to determine the
procedure of measuring similar moving masses independently of any theory.
(A similar procedure in GRT relates to the "mass of a body in the gravitational
field": the distinction of the gravitation mass from inert one, as contrary
to its own postulate). And this procedure should be namely the measurement,
rather than re-calculation, for example, via the postulated formula for energy
or momentum again. Otherwise the theory will try to "retain itself by the hair".
A similar measurement procedure does not exist for SRT.

The __physical__ concept of "mass" has no direct relation to all
those formulas (it is mathematics), which can include letter "". For the
basis concept of mass there exists the only clear - standard definition. It
determines the mass just at the state of rest (for example, the conditions
also exist for the standard of length - the temperature ones). And there is
no need to "invent a bicycle". In the motion the mass is simply
__not defined__, though letter can enter quite diverse formulas
containing , etc. These are different things! Therefore,
the definition of an elementary concept of mass in terms of more complicatedly
defined concepts of energy and momentum (depending on the theory,
interpretation, state of a system, etc.) is a physical nonsense (though,
possibly, it is correct mathematically). In such a manner one can "reach"
an absurd and define a simple notion of velocity as .
Note that any experiment, including measurement one, should be extremely clear
defined with respect to all conditions of its performing. And, generally
speaking, the "explanations" and "definitions" of theoretical physics
(for example, in SRT) often represent by themselves a drop-out from physical
understanding and a science-like masking of the essence of quantities behind
(often correct) mathematical transformations.