As you all know, I'm a Thomist. However, I see value in Descartes's famous maxim: cogito ergo sum, or "I think, therefore I am." In order to doubt that I exist, I would first have to exist in order to doubt it. Hence, my own existence is certain. However, I can also be certain that God exists and that God is distinct from me. Even if the external world were illusory, I experience change in my mind nonetheless, and hence I exemplify both actuality and potentiality.
As I've argued in the past, it is logically impossible for there to be an infinite regress of potentialities having their actualizations sustained. Therefore, we must arrive at some Pure Actuality: immutable, eternal, indestructible, unique, and omnipotent.
You can reject my arguments if you'd like, but I'm more certain of these two facts than I am that there exists an external physical world. What argument, after all, can be used to establish or even make plausible that the external physical world is real?