 This difference in genesis explains the different nature of the intellective form with respect to the other forms: whereas the vegetative and the sensitive forms are defined as acts of the body, the intellective form is the actuality of no part of the body and so is the act of the sensitive body in the sense of being its perfection: it is the completion of the process of development, by making it a full member of the human species. The relation of the intellective form to the body is that of the sailor to the ship (sicut nauta navi), but this union is accidental only from the point of view of its operations—such as in understanding, where the intellective soul does not need bodily organs (DOS 56)—but not from the point of view of the essence of the intellective soul (DOS 53).