Entry 0084: The Metaphysical Principles of ‘Essence’ and ‘Act of Being’
The expression ‘actus essendi’ is a technical term used by Aquinas in its restricted meaning. ‘Actus essendi’ is the metaphysical principle that goes ‘side by side’ with the metaphysical principle ‘essence’ in a subsistent extramental thing.
Three points of reference are indicated here. One, the real finite thing itself existing in the external world; another, the ‘essence’ which makes the thing to be what it is; and yet another, the ‘actus essendi’ which places both the thing with its ‘essence’ in actual existence.
In the real world ‘essence’ and ‘actus essendi’ are inseparable metaphysical principles. The metaphysical principle of ‘actus essendi’ always appears instantiated in an ‘essence.’ And the ‘essence’ of the thing is what put limits to the thing’s participation in ‘actus essendi.’
The doctrine of the ‘actus essendi’appears at every turn in the philosophical and theological writings of Aquinas.
Still Aquinas is emphatic in saying that the metaphysical principle of the ‘actus essendi’ is inseparable from ‘essence’.
At times Aquinas’ reflections concentrate more heavily and almost exclusively on the side of the metaphysical principle of ‘essence,’ but often his reflections rely entirely on the metaphysical principle of ‘actus essendi.’ Nevertheless, throughout his writings, Aquinas crosses from the plane of ‘essence’ to the plane of the ‘actus essendi’ and vice versa with remarkable facility.
The task of disentangling the nuances in doctrine he thus generates is not an easy one.
For Aquinas, the ‘act of being’ is the most profound perfection of a thing; it is an internal incommunicable metaphysical principle inseparable from the thing itself, from the ‘essence’ of the thing, and from anything that exists in the thing. No ‘essence’ actually present in nature makes itself known to the intellect without simultaneously making known its proper participation in ‘act of being.’