This seems puzzling : Isn't it so that ' Cassio is sad ', which proposition also expresses a state in which Cassio finds himself to be in, is nevertheless not a real relation? Indeed, this is true. Why then is ' Cassio loves (Desdemona) ' a real relation?
The reason is that ' Cassio is sad ' expresses an identity. And indeed this identity cannot be the case with respect to the real entities to which the terms ' Cassio ' and ' sad ' refer. It is only the terms together with their designation that are identified. So the relation as expressed by ' Cassio is sad ' cannot be a real relation, but only a being of reason. Indeed this non-real relation ' Cassio is sad ' intends the real relation ' Cassio has sadness ' (Cassio possesses the form sadness). Here Cassio r e a l l y possesses something, instead of being something.
Also in ' Cassio loves (Desdemona) ' Cassio r e a l l y loves someone, instead of being someone. This real relation can be intended by the non-real relation, that is, the relation of identity : ' Cassio is a lover (of Desdemona) ', where ' Cassio ' is the subject, ' is ' the copula, and ' a lover (of Desmona) ' the predicate.