Whether a belief is considered to be a delusion or not depends partly upon the intensity with which it is defended, and partly upon the numbers of people subscribing to it.
Anthony StorrWith the exception of certain rodents, no other vertebrate except Homo sapiens habitually destroys members of his own species.
Anthony StorrThe word "jealousy" is often used as if it were synonymous with envy; but I think the distinction worth preserving. Jealousy is predominantly concerned with the fear of loss of something one possesses, envy with the wish to own something another possesses. Othello suffers from the fear that he has lost Desdemona's love. Iago suffers from envy of the position held by Cassio, to which he feels entitled.
Anthony Storr