From The Way toward Wisdom:
An Interdisciplinary and Intercultural Introduction to Metaphysics
Fr. Benedict Ashley, O.P.
(Notre Dame, IN: University of Notre Dame Press, 2006), 122-123
The subject of natural science is being, but only changeable being (ens mobile). Yet, in establishing its own foundations, natural science finds it necessary to explain the very existence of changeable being by concluding, first (with necessity) to the existence of an immaterial First Cause; then (problematically) to the strictly immaterial nature of human intelligence; and finally (with some probability) to the existence of superhuman spirits. These are all known a posteriori, that is, as unobservable causes reasoned to from observed effects.
In comparing, as it must, the material things that are effects of the First Cause to that Cause, natural science must conclude that material things exist only contingently, since their existence depends wholly on the First Cause that alone exists necessarily. Thus the famous Thomistic “real distinction between essence and existence in creatures,” mentioned in the last chapter, is first manifested in natural science, although its implications for a full understanding of the Creator in whom essence and existence are claimed to be identical remains outside the scope of natural science. Within natural science, what must be kept in mind is that existence is proportional to essence, so that the question of “Does it exist?” always leads to or implicates the question “What kind of a thing is it?” since “whether" and “what” are correlative, like form to matter. As the form and matter of a changeable being condition each other, so do its existence (esse) and its essence. Its existence is an act by which its essence is real in a certain way, and its essence limits the manner in which it is real. Both a molecule of water and a human person “exist" as substances in our changeable world, but their activities are very different and a “thing is as it acts.” Substances that have more complex actions have more complex essences and correlated existences. [...] The act of being (actus essendi, esse) of a contingent being is, in the case of material beings, given to its matter through its actual form (forma dat esse was the Latin tag). This form has been made actual in the matter by the efficient cause of the being. In the case of a contingent but spiritual being, the actual form is identical with its substance. Thus, implicitly at least, Aristotle is always treating of things in their actuality or esse, in their “being as being,” just as is Aquinas more explicitly. The difference between what they have to say concerns the further development of the original point of view, the normal course of development in philosophy.
Since not only the kind of existence that material things possess but also the kinds of existence that the human spiritual soul and pure spirits possess are all effects of the First Cause, whose own existence we know through these effects, they are contingent, that is, do not necessarily exist. The First Cause, however, as is especially clear from the third way of proving its existence, since it is uncaused, cannot have merely contingent but must have necessary existence—that is, its existence and its essence must be identical, while the essence and existence of all contingent things are really distinct. This raises questions that are not the proper task of natural science to answer, but must be left to First Philosophy once it becomes clear that not all existent things are material.
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