The quantum relationship <img border=0 width=72 height=25 src="/fbpe/img/ingeniare/v16nespecial/image11-1.gif">may be regarded as the equivalence between two expressions for the rest energy of the particle, if <img border=0 width=17 height=26 src="/fbpe/img/ingeniare/v16nespecial/image11-2.gif">is considered as the spin angular velocity of the particle in its rest frame. The invariance of the relativistic space-time interval <img border=0 width=95 height=26 src="/fbpe/img/ingeniare/v16nespecial/image11-3.gif">to such a spin motion (space isotropy) leads to the spin momentum <img border=0 width=53 height=30 src="/fbpe/img/ingeniare/v16nespecial/image11-4.gif">for all structureless particles irrespective of their mass values. The inertia is an intrinsic property due to the spin motion of the particles. The signs of the mass values occurring in the solutions of the Dirac equation might be related to the orientation of the spin motion, as suggested by the fundamental relationship <img border=0 width=88 height=28 src="/fbpe/img/ingeniare/v16nespecial/image11-5.gif">. Besides it deals with the electron, and more specifically with two key properties: its complex wavefunction, and its intrinsic spin. In the standard interpretation, there is no clear real-space picture of what is oscillating in the wave, or what is rotating in the spin. Indeed, it is generally believed that no simple model of rotation can account for the spin of the electron. On the contrary, the present paper shows that a crude mechanical model of coherently rotating vortices can account quantitatively not only for spin, but also for the wavefunction itself. The implications of this are discussed in this paper.