 Each of these three forms has powers through which it performs the operations that are proper to it. Powers are defined by their actions, which in turn are defined by their objects. The powers of the soul—for example, memory or intellect—are active potentialities; once activated by an external agent they bring about their own actualities by way of their operations—for example, understanding in the case of the intellect. This conception of powers means that a human being performs different kinds of operations via different forms: a human being understands by means of a power belonging to the intellective form, not to the sensitive or vegetative forms (E 5, 43). Essentially different operations are rooted in powers belonging to essentially different forms; for example, perceiving requires bodily organs and understanding does not (QLIIS 8, 29; E 7, 51). As the result, human beings must be composite of essentially different forms.