[AntonioLao]

The outcry of quantum spin for its unintentional misunderstanding is contained in the 1st paragraph of Chapter VIII, Volume 3. 3rd Edition, Course of Theoretical Physics, Quantum Mechanics (Non-relativistic Theory) by L. D. Landau, et al, Pergamon Press, 1958 and quoted:

“In both classical and quantum mechanics, the law of conservation of angular momentum is a consequence of the isotropy of space with respect to a closed system. This already demonstrates the relation between the angular momentum and the symmetry properties under rotation. In quantum mechanics, however, the relation in question is a particularly far-reaching one, and essentially constitutes the basic content of the concept of angular momentum, especially as the classical definition of the angular momentum of a particle as the product of r ´ p has no direct significance in quantum mechanics, owing to the fact that position and momentum cannot be simultaneously measured.”