Aristotle maintains in the Posterior Analytics that it would not be possible to know the essential nature of anything and yet be ignorant of its existence: “Thus it follows that the degree of our knowledge of a thing’s essential nature is determined by the sense in which we are aware that it exists” (Post. Anal., 93a 28.) Such essential natures are known without mediation, i.e., without demonstration. Their existence, likewise, is known without mediation—as self-evident.
Our knowledge of the existence of any individual thing which comes under the purview of our immediate experience is self-evident. To Aristotle, “it would be absurd to try to prove that nature exists” (Physics, 193a 8.)
(Ralph J. Masiello, “A Note on Essence and Existence,” The New Scholasticism 45 : 491-494.)
It should be noted here that while actus essendi is inseparable from existence, existence as such is separable from actus essendi. There are in the external world things which are self-subsisting and things which are not self-subsisting.
The actus essendi of a self-subsiting extramental thing is what causes the thing—and everything in it—to stand actually present in the real world. And the presence of a self-subsiting extramental thing in the real world, the fact of its existence, is inseparable from the actus essendi of the thing.
In the real world, however, there are many things which are not self-subsiting. Things which are not self-subsisting, despite the fact of their actually existing in the real extramental world and that their actuality is real, do not possess actus essendi. And yet their existence is a fact.
The point I want to stress is this, that the connotation of existence is self-evident in both, in things which are not self-subsiting things and in the self-subsiting extramental things of nature. And more importantly, that in the self-subsiting extramental things of nature not only is existence self-evident, their actus essendi is also self-evident.