Actus of essence is form, and potentia of essence is matter. The form means definability of "what is", and plays the roll of determining principle ; the matter means indefinability of "what is" and plays the roll of limitting principle. According to the tradition since Pythagoras definability is "the determination by one" and indefinability is "the determination by two," or in other words, limitation. When we shift our attention from the dimension of essence to that of existence, actus and potentia, the fundamental concepts in metaphysics, come to relate themselves existence, and as a result our concern transfers to the division between actus of existence and potentia of existence. Actus of existence means esse simpliciter i.e. actus essendi, and this is existence in proper sense and is existence as determinig principle. On the otherhand, potentia of existence means esse quid as essence (whatness) i.e. modi essendi, categories of being in wider sense, and usually these are represented by the category of substance. Whatever hylo-morphological structure of substance might be, that is, either more material or more formal, the substance remains to be potentia against existence. As existence of a being proportionates to its essence, the essence itself constitutes an indispensable momentum of the existence. Not that the existence follows the essence, the essence follows the existence. Though the essence limits and conditions the existence, the essence cannot play the roll of determining in the place of existence. The roll of the essence in limitting and conditioning the existence must be preceded by the roll of existence to determine. The existence, after all, maintains its primacy over the essence. The distinction between existence and essence is real. Distinctio realis consists not only in the difference of things but also in a case, where the reason of distinction is recognized from the side of thing (a parte rei). Therefore there is exactly the same kind of distinction between existence as actus of existence and essence as potentia of existence, as that between form as actus of essence and matter as potentia of essence.
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