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Although there are many proposals of relativistic spin observables, there isno agreement about the adequate definition of this quantity. This problemarises from the fact that, in the present literature, there is no consensusconcerning the set of properties that such an operator should satisfy. Here wepresent how to overcome this problem by imposing a condition that everyoneshould agree about the nature of the relativistic spin observable: it must beintrinsic. The intrinsicality concept is analyzed in the relativistic classicallimit and then it is extended to the quantum regime, the spin problem beingtreated in the context of the irreducible unitary representations of thePoincar\'{e} group. This approach rules out three-vector proposals ofrelativistic spin observable and leads to a unique satisfactory spin definitionthat, besides being intrinsic, also possesses interesting physical featuressuch as covariance and consistency of predictions in the non relativisticlimit. To support the presented results from an operational perspective, aconsistent observer-independent model for the electromagnetic-spin interactionis also presented.

Relativistic spin operator must be intrinsic