  It is substantiated that spin is a notion associated with the group of
internal symmetry that is tightly connected with the geometrical structure of
spacetime. The wave equation for the description of the particles with spin one
half is proposed. On this ground it is shown that the spin of electron is
exhibited through the quantum number and accordingly the Dirac equation
describes properties of particles with the projection of spin equal plus or
minus one half. On contrary, we put forward the conjecture that the spin of the
quark cannot be considered as a quantum number, but only as an origin of a
non-abelian gauge field. It is a deep reason for understanding quark-lepton
symmetry and such important phenomena as quark confinement.
