The subdivision into id, ego and superego is related to that between Ucs. and Pcs. and is treated in a contradictory manner in the literature. Moreover, five different definitions were found concerning the boundary between id and ego. These problems are discussed with reference to the monographs by Gill, Arlow & Brenner and Schur. In agreement with these authors, it is concluded, on the basis of logical analysis, that the Usc.–Pcs. division is obsolete because it is contained in that of id–ego–suporego. Two of the five available definitions seem especially useful, among them the cybernetic one by Waelder, which has been unjustly ignored.