Act, Potency, and the Four Causes
Mobile being is divided according to act and potency. Potency, again, when speaking of mobile being, is regarded as matter or the material cause. Act is the formal cause, rendering the being in actuality and intelligibility. Potency, though, according to its own power, cannot become act. It must therefore become act by the operation of another. This other, which must itself be in act in some way in order to exercise power over that which is, of itself, in potency, is the efficient cause. No being, though, operates without some determinate end, all potency is ordered toward some act. This end is the final cause, that for the sake of which a potency becomes an actuality. So, from this division of mobile being into act and potency, we derive the four causes.