One crucial part of the Aristotle´s discussion of the principle of non-contradiction in Metaphysics IV 4 is his notion of semainein hen, which must be carefully distinguished from the closely connected notion of semainein kath´ henos. The Aristotelian proof is located at 1006b28-34 and it is argued that it does not suppose any bit of essentialism nor does it contain any petitio principii; some improvements are further proposed to the current translations.