Although there are many proposals of relativistic spin observables, there is no agreement about the adequate definition of this quantity. This problem arises from the lack of consensus concerning the properties that such an operator should satisfy. Here we present how to overcome this problem by imposing an unquestionable condition about the nature of the relativistic spin observable: it must be intrinsic. The intrinsicality concept is analyzed in the relativistic classical limit and then it is extended to the quantum regime to approach the relativistic spin problem. This approach rules out three-vector proposals of relativistic spin observables and leads to a unique satisfactory definition that, besides being intrinsic, also possesses interesting physical features such as covariance and consistency of predictions in the non-relativistic limit. To support the results from an operational perspective, a consistent observer-independent model for the electromagnetic-spin interaction is also presented.

尽管有许多相对论自旋可观观测量的建议，但对此量的适当定义尚无共识。该问题是由于对这种操作员应满足的特性缺乏共识而引起的。在这里，我们介绍如何通过对相对论自旋的本质施加一个毫无疑问的条件来克服这个问题：它必须是固有的。在相对论经典极限下分析了内在性概念，然后将其扩展到量子机制，以解决相对论自旋问题。这种方法排除了相对论自旋观测值的三向量提议，并得出了一个独特的令人满意的定义，该定义除了是内在的之外，还具有有趣的物理特征，例如在非相对论极限内的协方差和预测的一致性。