The word "stesso" is an intensifier of the reflexive pronoun "sé". "Sé" is the form used both for the third singolar person and the third plural person. The intensifier "stesso" needs to be accord in number and gender. So, if "stesso" strenghtenes a pronoun that stands for a singolar male entity, we'll have "se stessO", if the pronoun refers to a singolar female entity, we'll find "sé stessA"; if the pronoun substitutes plural things, we'll read for female "se stessE" and for male "se stessI". In this sentence, the subject is the verb "Giudicare" (to judge) and the objective complement is "se stessi". It's plural because the author is implicitly referring to a plurality of individuals that are judging. It would be also grammatically correct to use "se stesso", if we've implied a singolar individual: (Per gli individui) é molto più difficile giudicare se stessi che giudicare gli altri (Per l'individuo) é molto più difficile giudicare se stesso che giudicare gli altri The meaning is almost the same, at all.