A new formulation of statistical mechanics is put forward according to which
a random variable characterizing a macroscopic body is postulated to be
infinitely divisible. It leads to a parametric representation of partition
function of an arbitrary macroscopic body, a possibility to describe a
macroscopic body under excitation by a gas of some elementary quasiparticles
etc. A phase transition is defined as such a state of a macroscopic body that
its random variable is stable in sense of L\'evy. From this definition it
follows by deduction all general properties of phase transitions: existence of
the renormalization semigroup, the singularity classification for thermodynamic
functions, the phase transition universality and universality classes. On this
basis we has also built a 2-parameter scaling theory of phase transitions, a
thermodynamic function for the Ising model etc.