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In this brief note insightful remarks are made on the controversy on thedecomposition of the proton spin into the spin and orbital angular momenta ofquarks and gluons. It is argued that the difference in the perception on thenature of the problem is the main reason for the persistent disputes. There isno decomposition that simultaneously satisfies the twin principles of manifestLorentz covariance and gauge invariance, and partial considerations hide likelyinconsistencies. It is suggested that field equations and matter (i. e.electron in QED and quarks in QCD) equations must be analyzed afresh ratherthan beginning with the expressions of total angular momentum; canonical orotherwise.

Lorentz covariance and gauge invariance in the proton spin problem